Question: There are 3 switches downstairs, each corresponding to one of the light bulbs upstairs. How can you figure out which light switch goes to which light bulb by only making 1 trip upstairs?
Answer: You turn on one light bulb. Leave it on for a while. Then, turn that one off and turn another one on. Go upstairs. One light bulb will be cold, one will be warm, and one will be on.