There are three light bulbs in your attic. All of them are turned off and their switches are installed downstairs. You can play with the switches as much as you want and after that you can visit the attic above just once. How can you find out which switch to which bulb corresponds?
You turn the first switch on, then wait for 30 minutes and turn the second switch on. After that go upstairs and examine the bulbs. The one which is turned off corresponds to the third switch. The one which is turned on and is still cold correponds to the second switch. The one which is turned on and is hot corresponds to the first switch.