Stumbling Problems

A windowless room with three switches.

Many people are getting stumped by the problem,A windowless room contains three identical light fixtures, each containing an identical light bulb. Each light is connected to one of three switches outside of the room. Each bulb is switched off at present. You are outside the room, and the door is closed. You have one and only one opportunity to flip any of the eternal switches. After this, you can go into the room and look at the lights, but you may not touch the switches again. How can you determine which switch goes to which bulb? 
Answer: First, turn on one switch then wait for a few minutes. Then go into the room and feel the light bulb. The one that is warm is connected to the switch you turned on.