A windowless room has 3 lightbulbs. You are outside the room with 3 switches, each controlling one of the lightbulbs. If you can only enter the room one time, how can you determine which switch controls which lightbulb?

Turn on two switches (call them A and B) on and leave them on for a few minutes.  Then turn one of them off (switch B) and enter the room.  The bulb that is lit is controlled by switch A.  Touch the other two bulbs (they should be off).  The one that is still warm is controlled by switch B.  The third bulb (off and cold) is controlled by switch C.