A particular house has two rooms. If you're in one of the rooms you cannot look into the other. In one of the rooms there are three lamps which are connected to three switches located in the other room. For each lamp there is a separate switch. You are in the room with the switches. You are allowed to go only once in the room with the lamps. How can you find out to which lamp each switch is connected?

Turn two switches on. Wait some minutes and turn on of the two switches off. Then go quickly to the other room. The lamp which is on is of course connected to the only switch which is turned on at the moment. Touch the lamps witch are off carefully. The warm one was on recently, so that one is connected two the switch which was on for some minutes. The cold lamp is then connected to the switch you didn't change.