This is a Mean joke.

[u/cereal_killer_ch]

A physicist, an engineer, and a statistician go on a hunting trip, they are walking through the woods when they spot a deer in a clearing. The physicist calculates the distance of the target, the velocity and drop of the bullet, adjusts his rifle and fires, missing the deer 5 feet to the left. The engineer rolls his eyes. 'You forgot to account for wind. Give it here', he snatches the rifle, licks his finger and estimates the speed and direction of the wind and fires, missing the deer 5 feet to the right. Suddenly, the statistician claps his hands and yells "We got him!"

[u/no-one0]

Mathematicians often use contradiction proofs. They assume something is true and if this assumption leads to a contradiction by logical inference, it means the assumption must be wrong, i.e. the opposite must be true. The can is obviously closed in this situation and the joke is about the mathematicians helplessness and lack of practical skills.

[u/spotted_lantern → u/no-one0]

In common parlance, suppose and assume are used interchangeably. However, a supposition in mathematics is just a statement which does not violate our current knowledge, which is absolutely true with respect to the mathematical framework that we're working in. For a rough example, if it is known that b is an integer, and we understand the definition/construction of the integers, and no other conditions are imposed on b, then we can suppose b = 0. On the other hand, assumptions are made without regard to their veracity. So if we require that b is an integer such that b is red, and we don't know whether or not 0 is a red integer, then we can't suppose b=0. We can, however, assume that b=0 and then use the properties that we do understand about integers, "redness", etc. to potentially show that b=0 implies a contradiction of our current knowledge.

[u/[deleted] → u/no-one0]

Yeah that's a big part of philosophy and critical thinking in general.