I just saw this forum and thought I would add one. If it has been posted, sorry.
There is a room, with no windows. Outside the room are three switches. Inside the room are three plan light bulbs. Each switch goes to one fixture. You may do anything you want to the switches before you enter the room, but may not touch them after the door is opened. How can you tell, exactly, without guessing, which switch goes to which bulb?
