There are 2 rooms, one with 3 light bulbs mounted on the wall, one with 3 switches. Your goal: to figure out which switch goes to which light bulb.
These 2 rooms are separate and once inside one, you cannot see inside the other. The room with the 3 switches has 1 switch connected to 1 light bulb in the other room. There are no tricks to the wiring, each individual switch goes to exactly 1 light bulb, and vice versa.
You start out in the room with the switches. You can only go into the room with the light bulbs once. How do you find out which switch goes to which light bulb?
Turn on two switches, wait a minute, then turn off one of them. Then, go to the room with the bulbs. The lit bulb will be from the switch you left on, the unlit bulb that feels warm will be from the switch you left on for a minute then turned off, and the last bulb will be from the switch you didn't touch.
Perfect. Crocty's idea works too if you want to wait until one breaks/wear out :D