There is a closed room with a light in it. Outside, there are 3 light switches.
You can flick any of the switches any number of times. You can only open the door and go into the room once. You know that the light is initially off and you can't see what happens when you flick the switches.
How can you determine which light switch operates the light?
Leave switch 1 alone.
Flick switch 2 for 1 hour, then flick back.
Flick switch 3.
Now look. If the light is on, then the switch is 3, if the light bulb is warm, then switch 2, else switch 1.