Peter Winkler of Dartmouth College collects mathematical puzzles. Every once in a while, he encounters a particularly fiendish puzzle that gets him to scratch his head and wonder whether he heard it correctly. Or the puzzle sounds so trivial that he has to ask himself whether he missed something.
The following puzzle, which Winkler describes in the September College Mathematics Journal, belongs in the category: “Wait a minute—I must not have heard that correctly.”
The names of 100 prisoners are placed in 100 wooden boxes, one name to a box, and the boxes are lined up on a table in a room. One by one, the prisoners are led into the room. They may look into up to 50 of the boxes to try to find their own name, but must leave the room exactly as it was.