LOGICAL THINKING

There are three identical lightbulbs upstairs in a windowless room. Each lightbulb is connected to one of three switches downstairs. All bulbs are turned off at present. You are downstairs and cannot see into the room upstairs by any means. Before going upstairs, you may play around with the switches as many times as you like but once you go upstairs you may no longer touch a switch. After this, you go into the room and examine the lights. How can you tell which switch goes to which light?