there are 3 light bulbs in three seperate rooms. in front of you, there is a panel with 3 seperate switches. they only turn on or off. you may look inside each room only once. how can you tell which bulb belongs to which switch?
you turn on any two switches, leave them for a few minutes, and turn one off. you enter each room only once. you know that the lightbulb that is lit beongs to the switch that was left on, the bulb that is off, but hot, belongs to the switch you turned off, and the cold bulb belongs to the switch you never touched.