A physicist, an engineer, and a mathematician all get locked each into his own cell with no furniture and no nothing in it except a bucket of water, one can of food and a pencil.
The guardians leave the prisoners alone in their cells for three weeks. Then they open the cells.

First the physicist's cell. He is alive, the can is very much deformed, but empty.
The guardians asked the physicists how he opened the can to which he answered that he threw it again and again against the walls till it burst.

Next the engineer's cell. He is also alive. The can is empty, has lost its lid but is otherwise almost not deformed, and there are some formulas written on the wall.
The guardians asked the engineer how he opened the can. The engineer points to the formulas and explains that he calculated the optimal angle and force for a throw against the wall, and using this angle and force he opened the can at his first attempt.

Now it was time to open the mathematician's cell. They found the mathematician starved, the can untouched on the floor in the middle of the floor, but on all the walls were scribbled the words:




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Let's assume the can is open ...