Question: There are 4 light bulbs in a room and 4 switches corresponding to these four light bulbs (in a random order). You cannot see the light bulbs from where the switches are.
If you can only visit the room with the light bulbs once, how can you figure out which switch belongs to which light bulb?
Answer: Switch on two of the light bulbs for a long enough period of time for them to get hot. Then switch off one of those light bulbs and switch on one of the light bulbs that was off. At this point there will be a light bulb that is hot and on, hot and off, cold and on, and cold and off (allowing you to figure out which bulb goes to which switch).