Three switches, one bulb
Outside a closed room are three light switches. Inside the room is one ordinary bulb, wired to exactly one of the switches. You can flip the switches as much as you like, but you may enter the room only once.
How do you find out which switch controls the bulb?
Show hint
A bulb gives you more than one piece of information. Light is one. What else does a bulb produce?
Show solution
Turn switch 1 on and wait ten minutes. Turn it off, turn switch 2 on, then enter the room.
- Bulb on: switch 2.
- Bulb off but warm: switch 1.
- Bulb off and cold: switch 3.
The trick is using a second signal, heat, to get three outcomes from one observation.
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