You’re in a room with three switches and a door. Next door is a room with one light bulb, currently off. You can flip switches as you like, but you may only open the door once. How do you figure out which switch controls the bulb?
Think about other property of lightbulb that could give you additional information.
Idea of solution
Turn on switch A and leave it on for a few minutes so the bulb (if connected) has time to heat up.
Turn A off, then turn B on. Immediately open the door and check the bulb.
If the bulb is on → it’s switch B.
If the bulb is off but the bulb is warm/hot → it’s switch A.
If the bulb is off and cold → it’s switch C (the one you never touched).
This uses the bulb's light to detect a currently-on switch and its temperature to detect a recently-on switch.
