Three switches outside a room, one bulb inside, one trip in. Which switch is which?
A room has a single incandescent light bulb, controlled by one of three switches outside, all currently off. You may flip the switches however you like, but you may only enter the room once to observe the bulb. How do you determine, with certainty, which switch controls the bulb?
Reveal the answer
Turn switch one on, wait a few minutes, then turn it off. Turn switch two on and leave it on, then immediately enter the room. If the bulb is lit, switch two controls it; if it's off but warm to the touch, switch one does; if it's off and cold, it's switch three. The trick uses the bulb's residual heat as a second channel of information beyond its simple on/off state, a classic lesson in extracting more than one bit from a single observation.