Riddle corner (Q1, no answer)

I am doing a new thing called riddle corner! Every so often I will post a riddle. Click on the title of this post and answer in the comments. The next one will have the second question (Q2) and the answer to here (A1). Be the first to get it right and I'll mention your name (put your name in the comment, don't leave it anonymous) Here it is:

There are three light switches at the bottom of a staircase. At the top there is a door, and behind the door is a light bulb. The door lets out no light. Once you go up and open the door, you cannot go back down. How do you find out which light switch activates the bulb. (Only one does, and this IS NOT asking how to turn the light on. Turning on three lights doesn't tell you anything.)

Good luck!

P.S. Don't answer if you already know the answer.