You beat me to it. If you weigh the first 2 and it doesn't move, just keep one of them on it and weigh the rest. It'll only get used once if the first 2 weighed the same; when the see-saw finally moves and you automatically know who is heavier or lighter by the movement.

What if it's over thought and is actually just a riddle. This answer doesn't give you the "I'm right" feeling and plays around with the "use" of seesaw, but what if you put a guy on each side and they weigh the same … the seesaw never moves, so the seesaw was never used. Do this 10 more times and you eventually get…