A cruel tyrant puts a physicist, a hacker, a mathematician and a topologist (A more special kind of mathematician) into cells with only a tin, but no opener.
After a few days, he commands the cells to be opened:
The physicist has written a few formulas on the wall, and managed to cleanly open the tin by precisely throwing it against the wall.
The hacker has written a complete wall full of strange, confusing lines of code and formulas. The tin is ripped into tiny pieces, but the hacker is well-fed.
In the next cell is a dead mathematician. He has written on every inch of the walls in despair, the first line of his writing: "Assuming the tin is open..."
As they open the last cell, they find again many complicated formulas on all walls, but no dead or alive topologist. Instead there is only the closed tin in the center of the room. As they pick the tin up, there is a silent knocking sound and the high-pitched curse: "damnit, wrong sign!"