You have just walked into a dark two-story house. There are three light switches on the first floor A, B and C. Only one of the switches works, and it turns on the light on the second floor.
You cannot see the second floor light from the first floor and there are no windows in the building, but you do have a flashlight.
You can only make one trip up the stairs, and you can only flip one switch at a time. How do you determine which of the three switches turns on the light on the second floor ?
Turn on switch A, wait two minutes, then turn it off. Turn on switch B, then go upstairs. If the lights is hot, but not on, then switch A was the right switch. If the light’s on, then it’s switch B. If the light is neither hot nor on, then it’s switch C.