Yeah, it's another one of those "3 people" jokes

There was a mad scientist who kidnapped three colleagues; an engineer, a physicist, and a mathematician, and locked each of them in seperate cells with plenty of canned food and water but no can opener.

A month later, returning, the mad scientist went to the engineer's cell and found it empty. The engineer had constructed a can opener from pocket trash, used aluminum shavings and dried sugar to make an explosive, and escaped.

The physicist was still in the cell. She had worked out the angle necessary to knock the lids off the tin cans by throwing them against the wall. She was developing a good pitching arm and a new quantum theory.

The mathematician had stacked the unopened cans, and had a surprising solution to the problem. His dessicated corpse was propped calmly against a wall, and this was inscribed on the floor in blood:  Theorem: If I can't open these cans, I'll die.