Brain Teaser 1 - Light Bulbs

Ok, here is a brain teaser.

If you ant to, maramail me your answer and I'll tell you if you're right. I can also give you clues if you want...:

There are three light switches on the ground-floor wall of a three-storey house. Two of the switches do nothing but one of them controls a bulb on the second floor. When you begin the bulb is off. You can only make one visit to the second floor. How do you work out which switch is the one that controls the light?