There are two rooms. They have regular walls between them and are completely seperate, except for a door connecting the two. When the door is closed, it lets absolutely no light through. The first room has three light switches (Switch A, Switch B, and Switch C), each of which starts in the off position. The second room has a bare lightbulb, connected to the wall four feet up. One of the three switches turns on the light bulb. The other two do nothing. You do not know which one.
You start in the first room. You may flip any of the light switches, as many times as you want for as long as you want. But you may not open the door into the second room, yet. Once you've played with the light switches, you have to open the door and walk into the second room. Once you open the door you may not touch any of the light switches again. After you've been in the second room you must know EXACTLY which light switch turns on the light bulb (Switch A, B or C).
Hint: It doesn't matter if the light bulb is on or off when you enter the second room. That's not the point. You simply must KNOW which switch turns on the light bulb.