Three switches, one bulb
✓ With solution
Three switches outside a closed room, and only one of them works the bulb. You can go in just once: how do you find it?
Outside a closed room there are three light switches, numbered 1, 2 and 3, all switched off. Inside the room is a single old-fashioned bulb, the kind that gets hot when it's on, and only one of the three switches works it. You can't see the bulb from outside. You may flip the switches as much as you like, but you may go into the room only once, and you can't touch the switches after that.
How can you tell which switch works the bulb?
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The bulb can tell you more than just on or off. What happens to it after it has been on for a few minutes?
Show the answer
Answer: Turn switch 1 on for ten minutes, then off; turn switch 2 on and go in: on means 2, off but warm means 1, off and cold means 3. Turn switch 1 on and leave it for about ten minutes, so the bulb warms up if that's its switch, then turn it off. Now turn switch 2 on and go straight in. If the bulb is on, it's switch 2. If it's off but warm, it was on a little while ago, so it's switch 1. If it's off and cold, it's switch 3, the one you never touched. The idea: the bulb has three states you can tell apart (on, off and warm, off and cold), and three states are enough for three switches.
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