I remember playing with a differential engine in those Lego gear sets many many years ago. When you have a bunch of the pieces lying around and free time, you'll probably accidentally make a differential too.

As for this video itself, I've seem it a few times, and it still really does amaze me how much care was put into this. I assume the video is government funded, and it kind of makes me sad to realize that they had the capability to create educational videos of this quality 90 years ago, but it wasn't high enough priority to broadly apply to the general masses.