There are three light bulbs in a room, and three light switches outside the room.
You are outside, and want to match up which switch goes with which light bulb.
You can only travel into the room once, and cannot come back in again.
You can do anything you want upon entering the room.
How can you set the situation so that you will know
which switch goes with which light bulb?
Easily enough, just turn two of the light switches on.
Wait a while, and then turn one of them off, then quickly enter the room.
One light will be on, one off, and one off but still hot.