This 1937 video from Chevrolet explains one of the automobile's most important innovations: the differential. The premise is pretty simple. Back when cars were still a newfangled, futuristic contraption, engineers needed to devise a way to deliver differing amounts of power to each wheel during cornering. The way they got there, to the modern-day differential, is pretty fascinating. And if I'm not mistaken, this video explains a complex process better than just about anything I've ever seen.
[video:http://www.youtube.com/watch?v=yYAw79386WI&t=2m1s width:640 height:390]