BRAIN TEASER!!! (only answer if you DONT no it) thanks:-D?

you are in one of 2 rooms. the room that you're in has 3 light switches. one of the 3 turns on a light in the other room. you can only go to visit the other room once and you have to be positively CERTAIN which one turns on the light. how do you do it. and PLEASE don't give me ANY stupid answers thanks:-D