A girl is standing in a hallway with three lamp switches on the wall.
Each switch will turns on a different lamp inside a closed classroom. The girl can’t see inside the classroom, and she cannot open the door except to enter the room. She is allowed to enter the room only once, and when she do, all the lamps must be turned off.
How will tell which switch turns on which lamp when she enter the classroom?
Easy 🙂 She turn on the right switch and leave it on for two minutes. After two minutes, she turn on the middle switch and leave it on for one minute. When that minute is up, she turn off both switches and enter the room.
One light bulb will be hot (the first switch) and one will be warm (second switch) whereas the cold bulb will correspond to the switch she did not turn on.