In a large house there is a basement with three rooms. In the middle of each room is a light. There are no windows in any of the rooms and no light switches.
In the attic there are three switches. Each switch controls one of the lights in the basement.
You are taken to the attic. Your task is to try to determine which switch controls which light. Once you leave the attic, you may not return. You may visit two of the rooms in the basement once, and once only. No room, or the light emanating from it, is visible from any other room. You may not call upon the help of any other persons, or use any additional apparatus.
It is possible, but how?