An engineer, a physicist, and a mathematician
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An engineer, a physicist, and a mathematician (a topologist, to be specific) all volunteer for a psychology experiment. The experimenter puts them each in a room with no way out, and a can of food.
A few hours later he comes back. He peeks in the engineer's room. One wall is entirely covered with dents where the engineer threw the can against the wall, and the engineer is sitting in the corner, happily eating his food.
Next, he peaks into the physicist's room. One wall is entirely covered with equations scratched out, and there is one dent where the physicist threw the can at exactly the right angle to bust it open. Like the engineer, he is sitting in the corner, happily eating his food.
Finally, he comes to the mathematician's room. Like the physicist's room, one wall is entirely covered with equations, but strangely, there is no mathematician to be found. He steps into the room to investigate, and he hears a sound. It seems to be coming from the can sitting in the middle of the floor! He grabs a can opener and opens the can, and the mathematician pops out.
"How did you get in there?" the experimenter asks, shocked. The mathematician mumbles something under his breath, but the only words the experimenter catches are "sign error."