Riddles & Puzzlers

I've always Loved riddles and puzzlers. Here's one my son was presented with in school yesterday and he managed to come up with a solution in less than 30 seconds. Can you get it (in any amount of time) withOUT cheating by looking up the answer online?) I was not able to solve the puzzle. And please add your favorite riddle or puzzler. 

There are three overhead projectors in a school class room and three light switches in a room on the opposite side of the school, each of which is connected to one of three overhead projectors. (The switches are labled A, B, & C while the projectors are labeled 1, 2, & 3). Now you are in the room with the switches and you want to know which switch goes with which projector. You can only travel across the school to the room with the projectors one time, and you cannot come back to the room with the switches. You can do anything you want upon entering the room with the projectors. How can you determine which switch goes with which projector”