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You’re standing in a hallway with three light switches on the wall, each of which turns on a different lamp inside a closed room. You can’t see inside the room, and you can’t open the door except to enter the room. You can enter the room only once. How can you tell which switch turns on which lamp?
Turn on the 1st switch and keep it on. Then, turn on the 2nd switch and leave it on for one minute. When that minute is up, turn off the 2nd switch and enter the room. One light bulb will be on (1st switch) and one will be off, but still warm (2nd switch). The cold bulb will correspond to the 3rd switch, which you never turned on.