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?
Let us denote the three switches by A,, B and C. Then the individual should perform the following steps:
1) He should pick any of the switches, say A, and turn it on. Switch-A is kept on for a few minutes, and then it is turned off.
2) He should pick another switch, say B and turn it on.
3) He now enters the room with Switch-B turned on.
a) If one of the bulbs emits light, then B must be the switch connected to it. So, B is the correct switch.
b) If none of the bulbs are lit, he touches each of the three bulbs in turn.
b-i) If one of the bulb is warm then it must be connected to A, and so A is the right switch.
b-ii) If all three bulbs are cold, then by process of elimination, it follows that C is the switch that turns on one of the bulbs, although the bulb that C will turn on cannot yet be identified.
Consequently, this process will allow the individual to accurately identify which of the three switches will turn on the bulb.
Puzzle Solution
