I happened to be looking up another mystery (to me) - What's the difference between All Wheel Drive and Four Wheel Drive? The site I found explaining that had this video from Chevrolet. It was made back in 1937, it's not in color and it starts with a precision motorcycle team and you wonder, "How is this going to explain a differential for four wheel vehicles?" Well, just watch. It's very well done and makes it seem so simple.
In case you have trouble playing the video, here is the link to the original.
By the way, the original article I was looking for, explaining the difference between AWD and 4WD, is found at this link. It's really good, too.