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A classic of indirect observation: you can't see the light bulb while flipping the switches, so you need to leave a clue that doesn't just depend on whether it's on or off.
In a room there are three switches. In another room, which you can't see from the first, there is a light bulb connected to only one of them.
You can manipulate the switches however you want, but you can only enter the light-bulb room once. The bulb is one of those that gets hot when it is on for a while.
How do you determine for sure which switch controls the light bulb?
Answer: use light and heat. Turn on the first switch and leave it on for a while. Then turn it off. Turn on the second switch and enter the light-bulb room. Upon entering, there are three possibilities: - if the bulb is on, the correct switch is the second one;
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