Outside a closed room there are three switches. One controls a bulb inside. You may flip switches as you like, but you can enter the room only once. How do you find the right switch?
Hint: A bulb gives off more than light.
Turn switch 1 on for a few minutes, turn it off, turn switch 2 on, then enter. Lit = switch 2, warm and off = switch 1, cold and off = switch 3.
The trick is using heat as a second signal. A bulb that was on for a while stays warm after it's switched off, so you get three distinct states from one visit.
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