In the attic of your house are 3 lightbulbs, A, B, and C. The switches to these bulbs, 1, 2, and 3, are for whatever odd reason located in the basement of your house. Each switch is connected to only one lightbulb, but you cannot remember which. How can you figure out what switch connects to which bulb in only a single trip from the basement to the attic? I.e. you flip as many switches as you like, as often as you like, and then go upstairs and know for a fact which switch controls each bulb.
Flip switches 1 and 2 on, then wait 10 minutes and switch 2 off. Then, when you go upstairs, the light bulb that’s on connects to switch one, the light bulb that’s off and hot connects to 2, and the one that’s off and cold connects to 3.