Imagine you are in a room with 3 switches. In adjacent room there are 3 bulbs (all are off at the moment), each switch belongs to some bulb. It is impossible to see from one room to another.
How can you find out, which switch belongs to which bulb, if you may enter the room with bulbs only once?
Keep the first bulb switched on for a few minutes. It gets warm, right? So all you have to do then is... switch it off, switch another one on, walk into the room with bulbs, touch them and tell which one was switched on as the first one (the warm one) and the others can be easily identified...
