A conference room contains three separate wall-mounted spotlights - right, left and front of stage. Each is controlled by its own on-off switch. These three switches are numbered 1, 2 and 3, but they are in a back-room which has no sight of the the spotlights or the conference room (and there are no reflections or shadows or mirrors, and you are alone). How do you identify each switch correctly - right, left, front - if you can only enter the back-room once?