You are in a room with three switches labeled A, B, and C. Each switch controls one of three light bulbs in another room, which you can't see. All bulbs start off. You may toggle switches as you like, but once you enter the bulb room you can't change them again. How can you determine which switch controls each bulb with only one visit to the bulb room?
Turn on switch A for a few minutes, then turn it off and turn on switch B. Enter the bulb room: the lit bulb is B, the warm-but-dark bulb is A, the cool-dark bulb is C.
