Imagine you are in a room with 3 switches. In an adjacent room there are 3 bulbs (let's say in lamps which are on a regular table), each switch belongs to one bulb. All are off at the moment. It is impossible to see from one room to another. No help from anybody else is allowed.
How can you find out which switch belongs to which bulb, if you may enter the room with the bulbs only once?
You only need to turn on 1 switch as each switch belongs to one bulb. Walk into the other room and remove another bulb.