If a room has one bulb and there are three switches outside the room, how can you determine which switch controls the bulb if you can only enter the room once, and only while one switch is turned on?

Question

Accepted Answer

The right key is the switch that is turned on.

The bulb will only turn on if the switch that controls it is turned on.
Since you can only enter the room once, you need to determine which switch is turned on without entering the room.
You can try each switch one by one and observe if the bulb turns on or not.

Accepted Answer

Solution-
First turn on switch1 for sometime...
Then turn off and enter room and check the bulb temperature.. if hot then switch1 is right key.
Else turn on switch 2 and enter if bulb glows then switch2 is right key else switch3