OK, if you already know this one, please don't ruin it for other people or anything. That would suck. If you don't know it but think you know the answer, then PM me. You get one guess. Well, you can have one guess per 30 posts in this thread.
Now. The rules are simple - Guess how it was done.
A man walks into a two-room shack. It's impossible to get from one room to the other without going outside - the rooms are not connected with doors. In one room, there are three identical light switches. In the next room, there are there identical lights. The man goes into each room once and straight away he knows which switch leads to each light. How does he know?