A light bulb is hanging in a room. Outside of the room there are three switches, of which only one is connected to the lamp. In the starting situation, all switches are off and the bulb is not lit.

You are allowed to enter the room only once (the bulb is not visible from outside). How can you determine which of the three switches turns on the light bulb?

I heard this problem before with 4 switches. 

Assume you have switches A B C D.  You flip switches A and B.  After some minutes you take off switch A and put on switch D.  Go into the room immediately and observe the bulb:

Lit and hot - B

Lit and cool - D

Unlit and hot - A

Unnlit and cool - C

Of course this assumes the bulb takes a while to get hot

Posted by Andre on 2005-09-02 18:00:52