You have three lights in a box, each of which is connected to a switch outside the box. The box is closed. You need to figure out which switch to corresponds to which light and you can open the box only once (and you can't manipulate the switches after you open the box). How do you do this?

Answer: Turn on a switch and leave it on for a while.

Then turn it off and turn another switch on.

Open the box.

Feel the lights - the warmest light corresponds to the switch that was first turned on for a while then off.

The light currently on corresponds to the switch that is currently on.

The remaining light and switch correspond to each other.