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?

Flip the first switch on and wait one minute. 

Flip the first switch off, flip the second switch on, and go immediately into the room.  Touch the bulb.  If the bulb is:

Off, but hot, it's the first switch.

On, it's the second switch.

Off, but cool, it's the third swtich.

Posted by Lisa on 2004-04-21 09:36:13