An engineer, a physicist, and a topologist are subjected to a psychological experiment. The experimenters put each one in a small, separate room with plenty of canned water and canned tuna fish... but no can opener, only pencil and paper to work with. The subjects are locked in and then left to their own devices for a week.
When the week is over, the psychologists return. First they open the engineer's room. The pencil and paper are unused, but the walls are heavily dented, and the engineer is eating happily from a can of tuna fish. He threw the can against the wall until it broke open.
In the physicist's room, a few sheets of paper are covered in calculations, there is one deep dent in the wall, and the physicist is munching tuna. She calculated exactly how she could throw the can against the wall so that it would break open.
The topologist hasn't opened any cans; on the other hand, she's gone. In her place are piles and piles of paper covered with calculations, and all the full cans that were there at the beginning of the experiment. The psychologists search, bewildered, through the mess and come across a can that seems to be making strange noises. They open it--and out crawls a shaken-looking topologist, pencil in hand, covered in tuna fish. "Oops," she mutters. "I must have gotten the sign wrong again."