In this puzzle there are four rooms.
Room A
Room B
Room C
The Control Room
In the control room there are three switches: switch 1, 2 and 3. Each switch controls one of the light bulbs in room A, B or C.
You are in the Control Room. You may turn the switches on or off. It is up to you. When you have done what you wish to the switches, you may then go into room A, B and C to see the effect. When you have visited each of the rooms you may not return to the Control Room. You will now know which switch operates which light bulb.
How is this possible?