FILE No.65Unsolved
Outside a basement door there are three switches (A, B, C). One of them controls a single bulb inside the basement. The bulb is inside the basement, and you cannot tell from outside whether it is lit.
All three switches are currently off. You may flip the switches as often as you like, but you may enter the basement only once. Once inside you can examine the bulb, including touching it.
An ordinary incandescent bulb gets hot after being on for a while, and it stays warm for some time after being switched off.
How can you be sure which switch controls the bulb with just one visit?
If you get stuck, open the hints one at a time, in order.
Can on and off alone tell three switches apart?
Could you use the fact that a bulb stays warm after being turned off?
Match three states to three switches.
How can you be sure which switch controls the bulb with just one visit?
Choose the answer closest to the truth.
Turn on A for a few minutes, turn it off, turn on B, then go in right away
The answer is: turn on A for a few minutes, turn it off, turn on B, then go in right away.
Leave A on for several minutes, then switch it off. Next, turn on B and enter the basement right away. Examine the bulb:
The key is not to rely only on on/off, but to use a third piece of information: temperature.
We think of a bulb as something that is only either lit or dark.
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