In case you're interested, here are two of the best video's I've seen describing how a watch works ( http://www.youtube.com/watch?v=OiCPu0SjEW4 ) and how a differential works ( http://www.youtube.com/watch?v=K4JhruinbWc&t=1m50s )

Don't let their old school-ness throw you off. They are both extremely well done and demonstrate in clear and simple language from first principles to finished object.