this old-timey video that actually makes me understand what a differential is and how it works. I think this link skips to the middle of the video where the explanation starts. Otherwise you have to wade through some curious Synchronized Motorcycle Police performances. Although they use the policemen circling in formation as a bridge to describe the problem that the differential addresses.
Can you believe I'm talking about a differential? Ha! Oh, the picture isn't a movie. The link is the movie. Have a nice day, y'all!