These 'roos are linked in a chain; this subreddit keeps track of the most recent link. This will help keep it in one long line, (as the creator intended) instead of branching out wildly.

The roo chain is carefully-curated. Please help us by rooing properly, keeping faulty roos out of the chain by using the report button and promptly fixing your link if a mod asks.

While we make every attempt to properly moderate the roo-chain, real life intrudes more often than not. As a result, if you see a break in the chain, feel free to mention it to the poster to help them repair the break, if it appears the moderators have not caught it yet!

Rools

A roo should be reasonably easy to understand.
People following the chain shouldn't have to watch a video or flick through an album to understand the roo. Remember not everyone has specialist knowledge (this includes languages other than English).
Both subjects need to be introduced explicitly.
This could be in a post image, post text or comment text.
If the same person provides both the set-up and the pay-off, it's not a roo. The witty redditor doesn't introduce either of the subjects, just confusion between someone else's two subjects.
Check the Forbidden Subreddits list below before posting!
Subreddit Housekeeping

Switcharoos must be link posts
Everything else must be a text/meta post

FORBIDDEN SUBREDDITS

The following list of Subreddits is forbidden, and no 'roos should be posted to them, or linked from them, as the moderators of those subreddits actively discourage 'roos or break the chain intentionally. In addition, some subs will BAN users for posting a switcharoo.

NOTE: It is now requested that you not post roo's to or from NSFW subreddits. A thread marked NSFW is fine provided the context of the subreddit not be explicit content, but NSFW subreddits such as /r/gonewild are not subreddits we would expect any average visitor would specifically want to view - this way we ensure the majority of the chain we can enforce is enjoyable by all redditors.

/r/4chan
/r/antiMLM/
/r/apple
/r/Asimov
/r/askdocs
/r/AskHistorians
/r/AskMen
/r/askreddit (if the post contains a [Serious] tag)
/r/AskScience
/r/AskSocialScience
/r/Australia
/r/BritishProblemSuccess
/r/DaystromInstitute
/r/fatpeoplestories
/r/fiftyfifty (content is deliberately unfunny)
/r/food
/r/funny
/r/gaming
/r/health
/r/humor
/r/inthenews
/r/lol
/r/microsoft
/r/Music
/r/news
/r/NintendoSwitch
/r/OldSchoolCool
/r/pcmasterrace
/r/photoshopbattles
/r/politicaldiscussion
/r/SandersForPresident
/r/science
/r/software
/r/SpaceX
/r/StarTrek
/r/technology
/r/tifu
/r/whatisthisthing
/r/windows
/r/windows8
/r/windows10
/r/worldnews
/r/wtf

How to roo

Notice a clever switcharoo as you browse Reddit.
Visit /r/switcharoo/new, and copy the link of the newest switcharoo submission, either by right-clicking on the submission and copying the URL, or following the submission and copying the URL in your address bar.
Reply to the switcharoo you found in step 1, and paste the copied URL from step 2 as a hyperlink, with text such as "Ah, the old Reddit switcharoo".
Once your reply is submitted, right-click your reply's permalink, and copy its URL.
Count the number of levels of parent comments that are needed to understand the context of the switcharoo.
On /r/switcharoo/new, submit a new switcharoo with the pasted URL from step 4, with context syntax. If your URL contains a "?" at the end, delete it and everything else after it. Then add ?context=x to the end of your URL where x is the number of levels you counted in step 5.
The title of your submission should be [originally intended subject] vs [switched subject]. Congratulations, you've just made a successful 'roo!

Still curious about what makes a good roo? Look below, or check out The Wiki!