Answer #42 : One bulb Three Switches Puzzle.

Q: You are standing outside a closed room.
You can see three switches on the wall.
One of them corresponds to the bulb inside the room.
You can leave the switches in any position you want.
There is no way to see the light inside the room from outside.

If you are allowed to go inside the room only once,
How can you identify exactly the switch which controls the bulb inside?

A:
Keep the first switch ON for 5 minutes. (If it is the correct switch, by this time the bulb will be warm.)
Switch OFF the first switch and ON the second switch.

Now go inside the room and see the bulb.

1.  If the bulb is ON, then the second switch is the correct one.
2.  If the bulb is OFF, check whether it is warm or not. 
3.  If the bulb is warm, then the first switch is the correct one
4.  If the bulb is not warm, then the third switch is the correct one.