Psychologists subject an

Psychologists subject an engineer, a physicist, and a mathematician – a topologist, by the way – to an experiment: Each of them is locked in a room for a day – hungry, with a can of food, but without an opener; all they have is pencil and paper.
At the end of the day, the psychologists open the engineer’s room first. Pencil and paper are unused, but the walls of the room are covered with dents. The engineer is sitting on the floor and eating from the open can: He threw it against the walls until it cracked open.
The physicist is next. The paper is covered with formulas, there is one dent in the wall, and the physicist is eating, too: He calculated how exactly to throw the can against the wall, so that it would crack open.
When the psychologists open the mathematician’s room, the paper is also full of formulas, the can is still closed, and the mathematician has disappeared. But there are strange noises coming from inside the can…
Someone gets an opener and opens the can. The mathematician crawls out. “Damn! I got a sign wrong…”