Turn the first switch on for a few minutes, turn it off, turn the second on and go in. Lit means the second switch. Dark but warm means the first. Dark and cold means the third. Try it with an LED and it barely works, because they hardly warm up.
Outside a closed room are three switches. One of them works the single old-fashioned light bulb inside, which is off. You can flick the switches as much as you like, but you can only go into the room once, and after that you can’t touch the switches. How do you find out which switch works the bulb?
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A bulb tells you more than whether it is on or off.
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Use the heat of the bulb
Source: Traditional, origin unknown