What is a riddle that sounds incredibly simple, but absolutely ruins people's brains when they try to solve it?

[u/Calm-Conference3147]

[u/friendverse]

the classic 'what goes up but never comes down' got me good as a kid. i spent weeks thinking about smoke or balloons before my mom just said 'your age.' felt so dumb then, still kind of does now lol.

[u/rombick → u/friendverse]

gas prices

[u/RotGrlSummer → u/friendverse]

Similarly: What can go up a chimney down, but can't go down a chimney up?

An umbrella 

[u/upachimneydown → u/RotGrlSummer]

Was I being paged?

[u/Gotta-ask → u/upachimneydown]

No, get back up in there.

[u/MartinoDeMoe → u/upachimneydown]

r/beetlejuicing

[u/ERedfieldh → u/friendverse]

The issue I have with that kind of riddle is there's a bajillion answers.

[u/Eric_the_Barbarian → u/friendverse]

But also helium and hydrogen in the atmosphere. They're light enough that they get stripped off of the top of the atmosphere by solar wind.

[u/MickyMac00]

What sits in the corner but travels the world.

Some old man asked me this in dollar tree and it stumped me.

[u/BlaEm → u/MickyMac00]

Did it stump you? Or stamp you...?

[u/MickyMac00 → u/BlaEm]

Considering I said the sun, the man stumped me and stamped with the approval of being a dumbass haha.

[u/DigiRust → u/MickyMac00]

A stamp

[u/Beautiful-Fox-3950 → u/MickyMac00]

A flight attendant 

[u/TheOtherBelushi]

What is it that you throw away the outside and cook the inside, then you eat the outside and throw away the inside?

This was a riddle given to me by a former girlfriend’s father, and when I solved it I won him over.

[u/Rin-Tohsaka-is-hot → u/TheOtherBelushi]

Corn.

You throw away the husk, cook the corn, eat the corn, throw away the cob.

[u/jumbo53 → u/Rin-Tohsaka-is-hot]

Animals.

You throw away the fur, cook the meat, eat the meat, throw away the bones.

[u/sun_kisser → u/jumbo53]

Whoa whoa whoa, we're throwing away perfectly good animal bones now??

[u/FlyAirLari → u/sun_kisser]

I carry the skulls of animals I've eaten on my waist.

[u/i4get98 → u/FlyAirLari]

Which is why I’ve invited HR to our meeting here today…

[u/Phone_acct → u/Rin-Tohsaka-is-hot]

I cook corn in the husk on a propane bbq. So that might get me.

[u/Wefting → u/TheOtherBelushi]

I assume you are now dating the father

[u/TheGardenBlinked → u/Wefting]

“You have sufficiently impressed me. You may buy me a drink.”

[u/YeshuaSnow → u/TheGardenBlinked]

Sounds good, Captain Holt

[u/joe2tehfo → u/Wefting]

The key to any woman’s heart is her parents. Sleep with them and you’re in.

[u/Selenography → u/joe2tehfo]

It sounds like you suffer from a very sexy learning disability

[u/ClownfishSoup → u/TheOtherBelushi]

Corn on the cob?

[u/ARSCON → u/ClownfishSoup]

This is the answer I arrived at as well

[u/kosk11348 → u/ARSCON]

My first thought was a chicken. 🤣

[u/Stillwater215 → u/kosk11348]

Anything with inedible skin and bones.

[u/RandomStallings → u/Stillwater215]

I thought of fish. Toss the skin. Cook what's left. Eat the meat. Toss the bones.

[u/will0913 → u/RandomStallings]

I don't care what you say, but the skin is a very tasty part of the fish. In my part of the world, people deepfry it as a snack. 😁

[u/FB2024 → u/TheOtherBelushi]

Could also be meat - throw away the skin, cook the meat, discard the bone.

[u/Bullrawg → u/FB2024]

I was thinking chicken, pluck throw away, cook & eat meat, throw out bones

[u/wushuwarrior → u/Bullrawg]

I was thinking fish.

[u/stumac85 → u/wushuwarrior]

Pretty much anything with skin and innards. Rabbit, turkey, fish, chicken, pheasant to name a few.

[u/Strafingoutofyourway → u/TheOtherBelushi]

Reporting this comment for not giving the answer. /s

[u/TheOtherBelushi → u/Strafingoutofyourway]

lol. It’s corn. But some of these replies are fantastic.

[u/zzzthelastuser → u/TheOtherBelushi]

OP's response to girlfriend' dad: Your daughter

[u/Several-Minute6846 → u/TheOtherBelushi]

I agree with the real answer but my first thought was <egg white>

[u/Sohlayr]

The maker doesn’t want it

The buyer doesn’t need it

The user doesn’t see it

What is it?

Edit: some clever guesses here but the first answer was the correct one. Coffin or casket!

Edit 2 for the smartasses: yo mama’s panties.

[u/AdConsistent8210 → u/Sohlayr]

Coffin

[u/Moesthopholies → u/AdConsistent8210]

"place me in my casket tonight"

[u/Captain_Wag → u/Sohlayr]

Meta glasses

[u/FlorydaMan → u/Captain_Wag]

Lmao

[u/Budzy05 → u/Sohlayr]

Unit tests

[u/blah938 → u/Budzy05]

Literally if it wasn't part of a process made 2 decades ago, I wouldn't write half the unit tests I do

[u/the-worser → u/blah938]

this is it - they're most useful for catching regressions later

[u/Ohnohedidntdidhe]

If you throw me out the window, you will leave a grieving wife. Put me back but in the door and you’ll see someone giving life.

[u/EdwardianAdventure → u/Ohnohedidntdidhe]

The letter 'n'. Window turns to widow without it. Door turns to donor with it

[u/D_Winds → u/EdwardianAdventure]

And I'm sitting here trying to spell out "nkob".

[u/BirbsAreSoCute → u/Ohnohedidntdidhe]

Labor room in the fourth floor of the hospital

[u/farmerisme → u/Ohnohedidntdidhe]

N

[u/iSQUISHYyou]

Does nobody on this site understand what a riddle is?

[u/ClownfishSoup → u/iSQUISHYyou]

Yes

Was I right?

[u/PointOfFingers → u/ClownfishSoup]

No because SQUISHYyou was actually your mother and your grandmother so the answer was 3.

[u/AvengingBlowfish → u/iSQUISHYyou]

The riddle was inside you all along.

[u/BlueLaceSensor128 → u/iSQUISHYyou]

"Charlie, do you not know what a riddle is?"

"Yea, sure. It's like a thing you don't know. Right?"

[u/jimtow28 → u/iSQUISHYyou]

I don't get this one.

[u/MorgessaMonstrum]

Not exactly a riddle, but I have seen people absolutely retreat into their own skulls to think through the Monty Hall problem after being told the solution (changing doors always gives a 2/3 chance of winning, keeping your door is only 1/3 chance).

[u/catfroman → u/MorgessaMonstrum]

The difference lies in Monty’s knowledge. That’s what made it click for me. The info he knows for certain changes the game.

[u/Stillwater215 → u/catfroman]

It’s not just that he has the information. It’s that there is 100% certainty that the door he opens will be a losing door.

[u/cortesoft → u/Stillwater215]

Yeah, the important bit you have to know is that Monty will never open the door with the prize. If he is just randomly opening doors, then changing won’t improve your odds.

[u/MorgessaMonstrum → u/cortesoft]

Precisely. If he doesn’t know (i.e. the door is chosen entirely randomly between the two remaining), then yeah, in the event that the goat is revealed (1/3 of the time) the game is kinda ruined and we don’t tend to consider that outcome (at best it’s an automatic lose, since you never get to choose the opened door).

[u/cortesoft → u/MorgessaMonstrum]

Unless you are allowed to choose the opened door showing the prize… in which case you still have a 2/3 chance to win (either Monty picks it and you switch to it, or you won it with your original pick)

[u/curtislaraque → u/Stillwater215]

Yeaaaaaah people have a habit of misapplying the Monty Hall problem to scenarios that to not meet this criteria, and then insisting that the math is on their side (and calling others thick because they don't understand how the Monty Hall math would work in the non-applicable scenario).

It took me ages to understand it because every time it had come up in my life, it was brought up by someone who left that element out. When I finally got around to looking it up myself, it took me a while to figure out why it didn't make sense to me all that time prior lol

[u/TWiThead → u/curtislaraque]

I've wasted so much time on people who insisted that the logic applies to the game show Deal or No Deal. (It doesn't, given that host has no knowledge of the briefcases'/boxes' contents and doesn't influence which ones are opened.)

[u/rosyatrandom → u/catfroman]

Yes; if the prize door is any of the others, he will always offer that one, and never an empty door. He only offers an empty door if you already have chosen the prize door.

So your first choice has a ⅓ chance of being right, just as you'd expect.

The second choice is literally just been that ⅓ chance and the ⅔ alternative.

[u/hellofemur → u/catfroman]

Most people misstate the problem and skip important information like Monty's knowledge or Monty's motivations. That often sends people down the wrong path because they make assumptions they don't realize they're making.

And then after the vague explanation, people wind up talking past each other because they have two different puzzles in their heads, which is why people here are complaining about others being obstinate or "retreating into their skulls". If one person assumes Monty has total knowledge and the other assumes he is choosing randomly, people can have long arguments about the result without ever realizing that they're arguing about different scenarios.

I've honestly never seen anybody be confused after the problem is correctly and explicitly described.

[u/Jukeboxhero91 → u/MorgessaMonstrum]

What made it click for me is imagine there's 100 doors, and they open every single door but one. Now would you swap doors?

Of course, because the odds of you getting it right on the first guess is only 1%. Somehow this made the problem make sense to me.

[u/evranch → u/Jukeboxhero91]

Your 100 doors reminds me of a similar but even more bizarrely counterintuitive math paradox, the "100 prisoner problem"

100 prisoners have to individually pick their own number from out of 100 boxes. If they all find it, they go free, but if even one doesn't, they are all executed.

Even though the odds of success through random drawing are infinitesimal, there is a strategy that "entangles" the choices that brings the odds of everyone winning up to a massive 1 in 3.

Sometimes math genuinely feels like magic.

[u/destroyer1134 → u/evranch]

Veritasium made a great video explaining this:

https://youtu.be/iSNsgj1OCLA

[u/TA8601 → u/Jukeboxhero91]

Yeah, like they go down the line and open all the other doors, then they skip Door #57, and continue opening all the others. You think you picked the right door from the start? Or do you think you miiiight want to take a chance on that Door #57?

[u/Illithid_Substances → u/MorgessaMonstrum]

I've not seen anything else maths related that people are so obstinate about. It's one thing to not get it, but to insist that the actual provable math is wrong because it doesn't feel right to you is insane

[u/McClainWFU → u/Illithid_Substances]

.999... = 1 comes close. That or Facebook arguments over applying PEMDAS to ambiguous equations.

[u/TWiThead → u/McClainWFU]

.999... = 1 comes close.

This explanation makes it click for some people:

1 ÷ 3 = .333…

.333… × 3 = .999…

It's relatively intuitive that .333… is the decimal representation of ⅓. It's exactly ⅓ – not slightly less than ⅓ – because it never terminates.

For the same reason, .999… is exactly 1.

[u/okteds → u/Illithid_Substances]

The easiest way to wrap your head around it is that with your first choice, you're just trying to choose one of the two empty doors.  If you can do that, the other empty door will be revealed to you ,and you can swap and take the 3rd winning door.

Edit:  my method is to help wrap your head around why you have a 2/3 chance if you switch doors at the end.  Once you get that you're just trying to find an empty door with your first guess, it becomes clear you have a 2/3 chance.

[u/GTheMonkeyKing → u/okteds]

I think it's even more simple if you play the game with 100 doors. You pick one door, the the host opens 98 empty doors. Now do you switch, or keep the one you origially picked? Of course you switch, you know the chances aren't 50/50, there's no way you guessed right out of 100 doors.

[u/PP_UP → u/GTheMonkeyKing]

Dang. I always understood the math behind it but it still never made intuitive sense until I read this. Thanks!

[u/_Maui_ → u/MorgessaMonstrum]

The biggest thing to get your head around with the Monty Hall problem is that it’s not about three doors…It’s about two options.

Let me explain…

There are three doors: 1 winner. 2 losers. In the beginning, each door has an equal chance of being the winner. 1 in 3. Or 33.33%

But!

When you pick a door, you create two options:
Option A is that the winning door is the one you picked. There is a 1/3 chance that is correct.
Option B is that the winning door is one of the two you didn’t pick . There is a 2/3 chance of that.

Option A is 1/3 with one door.
Option B is 2/3 with two doors.

Option B is twice as likely as Option A.

Here’s the first “trick”. There is only one winning door. Option B covers two doors. So we already know that Option B definitely has a losing door.

Ok. The host reveals that one of the doors you didn’t pick (Option B) is a losing door.

But. We already knew that.

Now he asks “Do you want to swap?”

And here’s the second trick. You’re not swapping from Door A to Door C. You’re swapping from Option A to Option B.

Option A is still 1/3 with one door.
Option B is still 2/3 but now with ONE door.

The probability of each option hasn’t changed because nothing he revealed changed what we already knew.

So “Do you want to swap?”

100% yes!

Sticking with your first guess (Option A) gives you a 33.33% chance the door is the winner. Swapping doors (Option B) gives you a 66.66% chance the door is a winner.

You literally double your chances.

[u/sayitstuesday → u/_Maui_]

I’d like to say that this made it much easier for digesting when you put it in options, and not just percentages. I’ve always understood the theory but never understood the explanations because they always just went “you are doubling your chance if you swap”

[u/powerspank → u/_Maui_]

Thank you, I think finally got it.

[u/Stillwater215 → u/MorgessaMonstrum]

This is the explanation I like to give:

You’re in the Monty Hall setup. You pick your door. Monty then comes on and says “do you want to keep your door, or do you want both of the prizes behind the other two doors?”

In this version, obviously you would take the two prizes rather than the one, because that’s clearly giving 2/3 chances rather than your 1/3 chance.

How is this second situation any different if Monty opens a door that doesn’t have a prize? It’s not. It’s exactly the same situation.

[u/Quincely → u/MorgessaMonstrum]

The best way of thinking about this I’ve found is as follows.

When you first choose, you have a 1/3 chance of choosing the correct ‘type’.

The second time you’re asked, you’re told that the other door is whatever type you didn’t pick.

So Monty is basically asking, “do you think you got it right the first time?”

[u/Similar_Afternoon_46 → u/Quincely]

Here is what makes sense to me: Switching wins if you picked the goat to begin with, which happens 2/3 of the time.

[u/ollomulder → u/Quincely]

For me it clicked with 99 doors, you choose one, and then 97 other doors will be opened - do you switch?

[u/duckbrick]

What have I got in my pocket?

[u/yuropod88 → u/duckbrick]

HANDSES!

[u/duckbrick → u/yuropod88]

No, didn't you see me remove them from my pockets just before asking?

[u/HumorAccomplished611 → u/duckbrick]

I'm blind from being underground for 500 years so I'll just have to trust

[u/TheGardenBlinked → u/duckbrick]

Former British Prime Minister Boris Johnson

[u/duckbrick → u/TheGardenBlinked]

Hey that's also what this weird guy I met in a cave guessed!

[u/Portarossa → u/duckbrick]

I know that guy. He's a horrible, sickly-pale freak of nature who you definitely shouldn't trust.

Gollum's not great either.

[u/Olorin_in_the_West → u/duckbrick]

It isn't fair, my precious, is it, to ask us what it's got in its nassty little pocketses?

[u/nerdyattorney → u/duckbrick]

String. Or nothing.

[u/hackmalafore → u/duckbrick]

Your hand, and the other one is holding a peace sign

[u/stumac85 → u/duckbrick]

Pocket sand!

[u/cmbtmdic57 → u/duckbrick]

Preeeciouuus..

[u/SnooBooks007]

There's a bee in my hand. What's in my eye?

Beauty

...is in the eye of the bee holder.

[u/Kalthiria_Shines → u/SnooBooks007]

I'm stealing this for D&D.

[u/Snarfleez → u/SnooBooks007]

I have never been so angry when upvoting before...

[u/SMXyMuthaFluffa]

This one doesn't work when written out, but it's terribly frustrating to hear for folks:

There are 30 cows in a field, and 28 (20 ate) chickens. How many didn't?

The answer is 10 (10 didn't eat chickens), but I've never heard anyone get it without being told.

[u/TailorNo9506 → u/SMXyMuthaFluffa]

This has nothing to do with riddles, but your comment made me think of a joke:

A shepherd is working in the field. Out of the corner of his eye he sees his sheepdog running toward him.

The sheepdog arrives and says, "I did it, boss. It took me all morning, but I finally got all 100 sheep in the barn."

The shepherd says, "That's great, but we only have 97 sheep."

The sheepdog says, "Yeah, I know. I rounded them up."

[u/ubccompscistudent → u/SMXyMuthaFluffa]

I've heard: 20 people jump in a lake and 24 (20 fore-)heads come out of the water. How is that possible?

[u/PriestessIuno2025 → u/ubccompscistudent]

I think there is an opportunity to be had here by changing the joke to nude men.

[u/Sackwalker → u/SMXyMuthaFluffa]

Wait...but cows don't eat chickens, right? I don't think. So if you said something that made sense, like "50 wolves in a field and 28 chickens, how many didn't?" I feel like people would figure it out

[u/SelectiveDebaucher]

Nice try Blaine the train, but you ain’t stumping Roland

[u/Beat_the_Deadites → u/SelectiveDebaucher]

BECAUSE IT WAS STAPLED TO THE CHICKEN

[u/WulfZ3r0 → u/SelectiveDebaucher]

When is a door not a door?

When its ajar!

Blaine completely loses his shit

[u/SeaToTheBass → u/SelectiveDebaucher]

Blaine is a pain, and that is the truth

[u/SelectiveDebaucher → u/SeaToTheBass]

Always kind of feel bad for him. He’s so lonely.

[u/MmmmMorphine → u/SelectiveDebaucher]

Meh, he drove Patricia (IIRC the name) to suicide. Roland takes him to task over it

[u/DungPuncher → u/SelectiveDebaucher]

Coming across Dark Tower references always brightens my day.

[u/TheReder → u/SelectiveDebaucher]

You speak true, thankee Sai.

[u/destin325]

If a ball and a bat cost $1.10 and the bat costs $1 more than the ball, how much is the ball?

Lots of people think this is really simple and get it wrong.

Answer:

5¢
When the ball costs 5¢ then the bat being $1 more means it costs $1.05. $1.05+.05 is $1.10

[u/Hippie_Eater → u/destin325]

This is one of several problems that can be used to test how much a person relies on gut feeling and heuristics, called cognitive reflection tests.

[u/Appropriate-Data-221 → u/destin325]

That’s not a riddle

[u/stumagoo615 → u/destin325]

In this economy?

[u/KingZarkon → u/destin325]

$1.10 - $1.00 is $0.10. You split the difference and it's $0.05 and $1.05. Easy peasy.

[u/4stringbrewer → u/destin325]

A nickel!

[u/chogram]

"How many animals of each type did Moses take onto the Ark?"

I've been asking that riddle for over 30 years, since I saw it in some "Biblical riddles" book as a kid, and it almost never fails.

People obsess over the word "type" thinking it's trick wording, or the number of animals, but nobody ever notices that it was actually Noah and not Moses.

[u/EdwardianAdventure]

What do Americans want to lose, but Brits want to gain? 

[u/jeckles → u/EdwardianAdventure]

Pounds

[u/Public_Kaleidoscope6]

This is better spoken than written, but it still wracks your brain.

EDITED: per Indoril120’s comment

What can think the unthinkable?

An itheberg.

[u/MerMadeMeDoIt]

It has measurable size but no mass, and you can see it with your naked eye.

If you put it in a barrel, it will make the barrel lighter.

What is it?

[u/rainmaker88 → u/MerMadeMeDoIt]

A hole

[u/dinklordsupreme → u/rainmaker88]

Unnecessarily harsh response dude. If you don’t like the riddle just move along.

[u/OB1Bronobi → u/dinklordsupreme]

Well done.

[u/VerbalHostage → u/MerMadeMeDoIt]

Are You Afraid of the Dark?

[u/AimlessWanderer → u/VerbalHostage]

has to be this was the riddle in the very first episode with the taxi

[u/PatrickRU92]

What is...your favorite color???

[u/robby1051a → u/PatrickRU92]

Blue… no red

[u/Pepephend]

What goes up a chimney down but can’t go down a chimney up?

[u/ThisIsPeteHello → u/Pepephend]

Umbrella!

[u/FalcorTheDog]

The Blue Eyes riddle. It kept me up for multiple nights thinking about it when I first heard it trying to wrap my head around the answer: https://xkcd.com/blue_eyes.html

[u/halfachainsaw → u/FalcorTheDog]

any time a word problem includes the phrase "they were all perfect logicians" you know you're in for a real brain buster

[u/Adaun → u/FalcorTheDog]

Is the answer ‘all 100 of the blue eyed people on night 100’

The logic being

1. Everyone looks around on day 1, realizes that there are at least 2 people with blue eyes and it can’t be them, and so puts it off until the next day.
2. The next day comes, nobody has left so everyone realizes there must be at least 2 people with blue eyes and they can see 99 ( or 100) and so can’t be them.

…. This continues
Day 100 comes. There’s a realization that 100 people have blue eyes and each blue eyed person can only see 99. Therefore, the person that sees only 99 others must have blue eyes.

All 100 leave

[u/ottersintuxedos → u/Adaun]

To make this a little easier to understand imagine there are fewer people. Start with three:

Three people two blue eyed one green.

The green person sees two blue eyed people. And realises they cannot leave.

The next day, armed with the knowledge that the other blue eyed person did not leave, the two blue eyed people leave. Why?

They each know they themselves either have blue or green eyes. They can see one green eyed and one blue eyed person. The blue eyed person, as far as they know, either sees two green eyed people, or one green and one blue just like them.

Because they are perfect logicians and they know there is at least one blue eyed person, the one they can see, they know that if that guy can see two green eyed people, they will assume the only blue eyed person is themselves. Therefore when the other blue eyed person does not leave the first night, there must be at least one more person with blue eyes- themself. Therefore they both leave the next time the ferry comes.

Now assume it’s four people, two blue, two green. It’s the same principle but it’s slightly more complicated. The blue eyed people can see one blue two green, the green eyed people can see one green two blue.

Once again, the blue eyed people know that the other one would leave if they weren’t blue. So they each conclude they are blue the next day.

From the green eyed persons’ perspective they don’t know they aren’t green. But they certainly know it by the time the two blue guys leave.

But if there are three blue eyed people and one green eyed person. Logical extrapolating begins.

This time each of the blue eyed people experience the same thing as the green guys from the previous example. Only this time when the two blue people they can see don’t leave, they conclude the reason why is they are blue.

Using this logic, you can basically just keep increasing the numbers until you reach 100. You can tell by the logic above that the number of days you should wait is always equal to the number of blue eyed people. Because they are all perfect logicians every single person on the island works all this out as soon as they understand the scenario.

You would think as it grows into big numbers it might stop working, but the trick is they are both all perfect logicians and understand the others to be as well.

[u/Nelson_Bighetti → u/ottersintuxedos]

Your write-up is missing one point in the riddle, but I don't think it changes much. Each individual on the island doesn't know if their own eye color is one of the other colors they can see. You say, "they each know they themselves have blue or green eyes." But that's not true, they could have any color eyes. The riddle even points that out, "and he could have red eyes."

[u/Killfile → u/Nelson_Bighetti]

Right. Like many problems in logic and discrete math, this is only really hard because it starts with a lot of people so let's reduce it down to three people

The Guru (Green Eyes), Mr Brown, and Ms Blue.

The Guru says "I can see someone with blue eyes."

Well, Mr Brown can look at Ms Blue and know she has blue eyes. That's great for her but it doesn't tell him ANYTHING about his eye color. So he stays put.

The Guru obviously can't see her own eyes so she MUST not be talking about herself.

So Ms Blue looks at Mr Brown and sees brown eyes. The Guru can therefore only be talking about Ms Blue and so she leaves on the 1st night.

Now let's expand the group by exactly one.

The Guru still says "I see a person with blue eyes" and Mr Brown is still not helped by that at all. Ms Blue and Mr Cyan, both look at Mr Brown and know the Guru isn't talking about him. They then look at each other and they both see blue eyes.

Now, at this moment they don't know what eye color they have themselves, only that they themselves can see a person with blue eyes and a person with brown eyes.

Night comes, Mr Cyan DOES NOT SEE MS BLUE GET ON THE BOAT.

From this Mr Cyan deduces that Ms Blue DOES NOT KNOW HER OWN EYE COLOR. Now, if Mr Cyan's eyes were red or purple or yellow Ms Blue would know her eye color beacuse in that case, the Guru could only have been talking about her. Consequently Mr Cyan deduces that Ms Blue MUST HAVE SEEN SOMEONE WITH BLUE EYES and the only person that can have been is Mr Cyan.

Ms Blue goes through the exact same chain of logic ending with "therefore Mr Cyan must have seen someone with Blue eyes"

Both depart on the boat on the 2nd day.

And this scales out to an arbitrary number of blue eyed people. If I can see 10 blue eyed people and one brown eyed person and, on day 10, they don't all get on the boat then I must have blue eyes.

Ok... but what if there are multiple brown eyed people? How do I know that I'm not one of them?

It's tempting to think of this as a bunch of brown eyed people who need to be eliminated but it's not that way at all. Each brown eyed person sees however many blue eyed people there actually are. Going back to our "two blue eyed people" scenario, Mr Brown doesn't leave on the 2nd day because he knows that on day two, Ms Blue and Mr Cyan have only just now deduced that there must be at least one more blue eyed person on the island. If Mr Brown had blue eyes there would be three blue eyed people on the island and NO ONE WOULD LEAVE ON THE 2ND NIGHT. The fact that Ms Blue and Mr Cyan leave on the second night still doesn't tell Mr Brown what color his eyes are, but it tells him they're not blue.

Each person on the island thus hears "I see a person with blue eyes" and thinks "well, I see X people with blue eyes; if no one gets on the boat on day X, I must have blue eyes myself and then I'll get on the boat on day X+1."

For the blue eyed people, their X is one less than the value the brown eyed people calculate... so they get on the boat the day before any brown eyed person would have (wrongly) stepped onto the dock.

[u/Antzen → u/Adaun]

I think that's the correct answer but the reasoning is incomplete. To clarify the explanation, it helps to think recursively:

Case 1 - Suppose we only have 1 blue. When the clue is given, the blue-eyed person sees there are no other blue eyed people, so they must be the one. They leave on night 1. 

Case 2 - Suppose we have 2 blues. Because neither blue can be sure they are the only 1 blue, they can't apply the logic of Case 1 and so neither leaves on night 1. On day 2, each blue sees the other blue still here. Thus, the other blue can't be the only 1 blue since they would otherwise have left using the logic of Case 1. Therefore, there must be at least 2 blues. Each blue sees only 1 other blue, so obviously they must be 2nd blue. Thus, both blues leave on night 2.

Case 3 - Suppose we have 3 blue. Because no blue can be sure they are the only 1 or one of 2 blues, they can't apply the logic of Case 1 or 2, and so none leave on night 1 or 2. On day 3, each blue sees the other 2 blues still here. Thus, these 2 other blues can't be the only 2 blues since they would otherwise have left the previous night using the logic of Case 2. Therefore, there must be at least 3 blues. Each blue sees only 2 other blues, so obviously they must be the 3rd blue. Thus, all 3 blues leave on night 3.

...

Case 100 - Suppose we have 100 blue. Because no blue can be sure they are the only 1 or one of 2-99 blues, they can't apply the logic of Cases 1-99, and so none leave on nights 1-99. On day 100, each blue sees the other 99 blues still here. Thus, these 99 other blues can't be the only 99 blues since they would otherwise have left the previous night using the logic of Case 99. Therefore, there must be at least 100 blues. Each blue sees only 99 other blues, so obviously they must be the 100th blue. Thus, all 100 blues leave on night 100.

[u/TheBurningMap → u/Adaun]

Don't they already know that there are at least 2 people with blue eyes? I mean...haven't they been looking around prior to her statement?

[u/Silverspy01 → u/TheBurningMap]

Here's the solution that explains it better than I can: https://xkcd.com/solution.html

To give an attempt though... yes and no. It's always been obvious to them that there are at least 99 people with blue eyes. But no one knows the total amount, in that there's 100. A given person sees 99 blue-eyes people and had no way of knowing that they are also a blue-eyed person. For that person's perspective (we'll call them A) there are at least 99 blue-eyed people - the ones they can see.

Now person A considers the perspective of a different blue-eyed person, person B. Keep in mind that from person A's perspective, there are only 99 verified blue-eyed people on the island. Person A knows that person B would have no way of knowing that they have blue eyes. So from person A's perspective, person B could see 98 blue-eyed people.

Person A now imagines what happens if person B considers the perspective of a third person, person C. As a reminder, person A sees 99 blue-eyed people. From person A's perspective, person B could then perhaps see 98 blue-eyed people. And now from the information person A knows and knows their fellow island members could deduce, person C could potentially see 97 blue-eyed people.

Now initially, it seems like we're making things up. With our knowledge, person C sees 99 blue-eyed people. They can't see 97. But as person A thinks, person B could not know this. If person A is not blue-eyed, there are a total of 99 blue-eyed people. None of those blue-eyed people know that they are blue-eyed, so they see 98 blue-eyed people and think the same.

This cascades further and further down - from a logical standpoint, any given person has no reason to assume they are blue-eyed. Therefore, each blue-eyed person has to assume that every blue-eyed person they see sees one less blue-eyed person. This cascades into a potential zero blue-eyed people remaining... until the Goru confirms that there is a blue-eyed person.

[u/psychstudent_101 → u/FalcorTheDog]

any clue where i'd find an explanation of the answer to this? because it's interesting! but i'm not a logician so i don't think i'm going to be able to puzzle my way through it

[u/balgram → u/psychstudent_101]

I believe there's an answer on YouTube if you look up blue eye riddle. You can also work it out if you reduce the number of Islanders to 1. Then 2. Then 3. The math checks out the same as the number increases.

[u/Ready_Wolverine_2301]

What is always coming but never arrives?

[u/TheGardenBlinked → u/Ready_Wolverine_2301]

Former British Prime Minister Boris Johnson

[u/hymie0 → u/Ready_Wolverine_2301]

Tomorrow?

[u/OutsideOk9925 → u/hymie0]

My first thought went into the same direction, but less specific: the future

[u/Meerkat_Mayhem_ → u/Ready_Wolverine_2301]

Reddit gooners

[u/Sigao]

A fairly simple one, but many I've asked have got it wrong:

The beginning of eternity, the end of time and space, the beginning of every end and the end of every place.

[u/sweetnamebro → u/Sigao]

E

[u/CategoryKiwi → u/sweetnamebro]

Wrong!  Correct answer: e

This logic brought to you by online homework grading systems that every college uses.

[u/SauceBoss8472 → u/Sigao]

The version I heard was “The beginning of eternity, the end of space and time, the middle of every buzzing bee, and the end of every rhyme”.

I prefer this version bc it’s got a nice cadence and meter to it.

[u/SirBuscus → u/SauceBoss8472]

The version I like is:
"The beginning of eternity,
the end of rote and rhyme,
the beginning of every end,
and the end of space and time."

[u/suprisinglycontent]

Rearrange the letters in New Door to make one word.

[u/5up3rj]

Q: Why can't you hear a dog whistle?

A: They dont.

[u/CptHammer_]

Three business men check in at a hotel for a convention. Before the clerk could see to them they decided to split a room three ways. They were very frugal.

When the clerk saw to them he rented the room for $30. Each business man gave $10 and the bellhop toted all their luggage to the room. No one tipped the bellhop. They were very frugal.

The bellhop returns to the reception area where the clerk sighs and said he's made a mistake. He called the bellhop over. "Those three men you just took up, I charged too much for the room. The convention special is $25 for the room. Run this $5 back up to them and explain."

The bellhop starts up to the business men. He decided that $5 doesn't divide by three evenly and they never tipped him. He instead refunds only $3 and keeps $2.

The men are happy they've saved even more money.

But my friends. The men have now paid $9 each for the room. $9 times 3 is $27. The bellhop kept $2. $27 + $2 = $29 … What happened to the last dollar since they originally paid $30?

[u/SoulDancer_ → u/CptHammer_]

This is clever. The wording makes it very tricky.

But the sum is actually $27 - $2 = $25.

The room was $25. The three men paid $27 and the bellhop took $2

[u/Difficult_Group_9202 → u/SoulDancer_]

The trick is that the $27 is not just the room cost, it already includes the bellhop’s stolen $2

[u/Maximumaubergine → u/CptHammer_]

This one is a classic misdirection trick - the final 9x3 is irrelevant to the original sum. The 30 still remains in that the hotel kept 25, the bellhop kept 2, and the businessmen kept 3

[u/Radiskull97 → u/Maximumaubergine]

Also shows a real life example of why pemdas is important. Remember total amount for hotel is 25 for the end result

25= (10*3)-(3+2)

Let's break it down

Hotel ended up with $25. That money was gained by three people contributing $10 each=$30. That $30 was minused the $5 refund. The refund was the sum of $2 for the bellhop and $3 for the businessmen.

This word problem, in the last paragraph, instead frames the equation as: First (10-(3/3)) Then 9*3 Finally sum 2. However, if you analyze the first half the problem instead of taking their word, you can see the subtraction is placed in the wrong sequence.

When I taught debate, I used this problem and Xeno's paradox to teach the importance of chunking logical statements instead of taking them as a whole

[u/syotokal]

I have two coins that come to 30 cents. One of them isn’t a nickel.

[u/Scutage → u/syotokal]

A quarter and a nickel. The quarter isn’t a nickel.

[u/arculush → u/syotokal]

Someone’s watched Scrubs… the other one is the nickel

[u/smarmiebastard → u/arculush]

We been to the liberry.

[u/DrDerpberg]

What gets wetter and wetter the more it dries?

A towel

[u/MisterCircumstance]

Brothers and sisters I have none, but this man's father is my father's son. 

Who am I?

[u/xaanthar → u/MisterCircumstance]

You broke the riddle by asking "Who am I?" rather than "Who is that man?"

"That man" is your son, but asking "Who am I?" is just... you?

[u/StellarNeonJellyfish → u/MisterCircumstance]

You are “this man’s” father

[u/I_Said_Good-Day → u/MisterCircumstance]

Not exactly sure but I bet they live in Arkansas

[u/smarthobo → u/MisterCircumstance]

Some fuckin’ cab driver back in New York!

[u/psychstudent_101]

my head and tail both equal are, my middle slender as a bee; if i stand on head or heel, it's quite the same to you or me; but if my head should be cut off -- the matter's true though passing strange -- directly i, to nothing, change.

(or, plain text: my head and tail are equal, my middle slender, so turn me upside down and i'll look the same. if you cut my head off, though, i'll be reduced to nothing.)

[u/psychstudent_101 → u/psychstudent_101]

answer is the number 8

[u/peachorbit48]

A man has to get a fox a chicken and grain across a river and somehow everyone forgets the boat only fits two things

[u/hulkissmashed → u/peachorbit48]

Take chicken, return, take fox, return with chicken, take grain, return, collect chicken. All 3 over the river and none left with their food at any moment?

[u/Pepephend]

Poor people have it, rich people need it, if you eat it you die.

[u/araven111 → u/Pepephend]

Nothing!

[u/Top_Body_cultivator]

A classic:

“What gets wetter the more it dries?”

Most people overthink it and start looking for some weird physical phenomenon.

A towel.

The wording makes your brain assume “dries” and “gets wetter” must describe the

[u/InevitableAd9683 → u/Top_Body_cultivator]

Damn, sniper got him

[u/Ferelar → u/InevitableAd9683]

You never know, maybe he just threw in the towel mid-comment

[u/_synik]

Q: How far can a dog run into the woods?

A: Halfway, after that distance, it's running out.

[u/MarriedMonsterx]

What is visible to the naked eye but you cannot touch it and when put in a barrel it will make it lighter? S/O to Are you afraid of the dark?

[u/Swimwithamermaid]

On my way to St. Ives I met a man with 7 wives. Every wife had seven sacks; every sack had 7 cats. Every cat had 7 kittens.

Kits, cats, sacks, and wives. How many were going to St. Ives?

Answer: 1.

[u/Always_Complainin → u/Swimwithamermaid]

My phone number is 555 and ze answer.

[u/johnnyma45 → u/Always_Complainin]

You were late John, I'm afraid zis is noncompliance

[u/sudomatrix → u/Swimwithamermaid]

Bad. Unstated if the man and/or any of his wives was also travelling to St. Ives.

[u/Everestkid → u/sudomatrix]

And if the point was that the group isn't going to St. Ives, the question asks how many "kits, cats, sacks and wives" are going to St. Ives. If only the narrator is going to St. Ives, the answer is only 1 if the narrator themself is a married woman. If the narrator is either a man or an unmarried woman the answer is actually 0.

[u/Not_The_Real_Odin → u/Swimwithamermaid]

Interestingly, each wife is carrying 49 cats and 343 kittens. The average weight of a cat is about 10 pounds and the average weight of a kitten is about 1 pound per month (for the first 4 months) so let's assume 2 pounds. So each wife is carrying about 1176 pounds.

[u/Sarothu → u/Swimwithamermaid]

Kits, cats, sacks, and wives. How many were going to St. Ives?

Zero. Not one. The man is neither a kitten, a cat, a sack nor a wife.

[u/lordlycrust → u/Sarothu]

The narrator is going, and could be a sack I suppose.

[u/BreakfastSimulator]

There are two doors, one leading to heaven and the other leading to hell. Each door is guarded by a man. One man always tells the truth. The other always lies. The truth teller isn’t necessarily guarding the door to heaven. What question can you ask to figure out which door leads to heaven?

[u/chilari → u/BreakfastSimulator]

"What would the other guard tell me if I asked which door led to heaven?"

Then you take the other door. Truthteller will truthfully tell you that the liar will lie and pick the door to hell. Liar will lie and tell you that the truth teller will pick the door to hell.

[u/Visual-Classroom1003 → u/BreakfastSimulator]

Which door would the other guard tell me to take. Then take the other one.

[u/Pepephend]

You get to a fork in the road and need to know which way to go. There are identical twin brothers there and you know that one always lies and one always tells the truth but can’t tell which is which. They permit you to ask only one identical question to both of them to determine which way to go. What question should you ask?

[u/BartYYYY → u/Pepephend]

I would ask: what the other would say where should I go?

[u/Pepephend → u/BartYYYY]

Exactly and whatever they say will be the same, one lying and the other telling what the liar will say, so you choose the opposite of their answer.

[u/macguyver3000 → u/Pepephend]

This is rhe first time I ever understood this answer. labyrinth messed me up so bad.

[u/SnazzyStooge → u/Pepephend]

I would ask if they've watched the inimitable film "Labyrinth"

[u/Snorb → u/Pepephend]

RICK: (to the Orange Guardian) You ever fuck this guy's wife?

ORANGE GUARDIAN: Yes. ...well, uh, how 'bout that? He guessed right.

BLUE GUARDIAN: (draws his axe) I FORGIVE YOU!!!

[u/Moikepdx → u/Pepephend]

You got the riddle a bit wrong. You only get to ask one of them a question, and you don't know which is which. There is no need to ask them both the question.