[BluSyn]

Seems there are many variations on this joke.

The one I know:

An astronomer, a physicist and a mathematician go for a walk in the countryside. They spy a black sheep in the distance.

The astronomer immediately proclaims, "All sheep are black!"

The physicist thinks for a moment and suggests, "Some sheep are black."

The mathematician ponders awhile and says, "There exists a sheep such that at minimum one of its sides is black."

[Gibbon1]

There is the joke where scientists lock an engineer, a physicist, and a mathematician each in a room with a bed and a can of soup. In the morning they open the rooms one by one.

In the first room the engineer is asleep and there is mangled open can of soup on the floor. They wake him up and ask about the soup can. And he says, I got hungry so I smashed the soup can against floor to open it.

Next room the physicist was sleep and the soup can open on the floor with the top cleanly removed. And equations scribbled on every wall. When the wake him up and ask about the soup can he proudly says, I got hungry so I figured out exactly where to hit the can so the top would fly off.

Upon opening the last room they find a bleary eyed mathematician holding the soup can saying 'assume the can is open'

[andreareina]

The joke is usually an economist saying, "assume a can opener": https://en.wikipedia.org/wiki/Assume_a_can_opener

[Gibbon1]

I think economist is waiting for the free market to sell him a can opener.

[names_are_hard]

I heard a variation of this that ended with the mathematician scribbling the same equations as the physicist and then stating "and this the problem reduces to a previously solved problem" and going back to sleep without bothering to open the can.

Instead of a can of soup it was a fire that broke out in their respective rooms, the engineer sprays fire extinguisher all over the room, the physicist sprays precisely at the exact point that will extinguish the fire, and the mathematician is satisfied knowing that the problem has a known solution.