Only one of them controls a lamp up in the attic. You can’t see the attic from the basement.
You may flip the switches as much as you like, but you can climb to the attic only once.
How can you tell which switch controls the lamp?
💡 Hint
Your hand can learn something your eyes can’t.
Reveal answer
Turn on switch 1 and wait about ten minutes. Turn it off, turn on switch 2, and go upstairs. If the lamp is on, it’s switch 2. If it’s off but warm, it’s switch 1. If it’s off and cold, it’s switch 3.
With one visit, “on” and “off” only give you two outcomes, which isn’t enough for three switches. Heat adds a third outcome: an incandescent bulb that was recently on stays warm for a while after it’s switched off. Leaving switch 1 on long enough to heat the bulb, then switching to switch 2, gives each switch its own unique result: on, off-and-warm, or off-and-cold.
Did you get it?
About This Puzzle
This is a famous logic puzzle that looks impossible if you only think about on and off. The trick is to use one more thing you can sense.
How to Think About It
One trip gives you one look at the lamp, and a lamp is either on or off. Two states can only tell apart two switches, not three. So you need a third kind of information that the lamp can give you. Think about what an old-fashioned bulb does when it has been on for a while.
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