The Man in the Elevator
In the evening, he gets into the elevator, and, if there is someone else in the elevator - or if it was raining that day - he goes back to his floor directly. Otherwise, he goes to the seventh floor and walks up three flights of stairs to his apartment. Can you explain why?
This is one of the best known and most celebrated of all lateral thinking logic puzzles. Although there are many possible solutions which fit the initial conditions, only the canonical answer is truly satisfying.
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The man is a of short stature. He can't reach the upper elevator buttons, but he can ask people to push them for him. He can also push them with his umbrella.