I'm crazy about these old instructional filmstrips. I don't know what it is about them, but for all our modern pedagogy and graphics technology I rarely see a contemporary demo that makes me sit up and go "OH! That's how that works!" the way these do.
This one about differential gearing is making the rounds in the last 24 hours:
Some previous favorites are this one about how a mechanical watch movement operates...
... and this series from the US Navy about mechanical computers. I knew they had these things called 'fire control computers,' and I knew they were made out of shafts and gears and other physical widgets, but I had never really thought about what they were trying to do, to say nothing of how they managed to actually do it.
Those are the first three clips. (NB The last one uses differential gears to do addition.) There are four more: four, five, six, seven.
By the way, does anyone else keep a mental list of things that would be important to remember if they get sent back in time? Sometimes I see something and think "remember that, it might come in handy one day." But sometimes I think "remember that, it might come in handy if you get transported into the past." The latter only carries slightly less weight than the former for me.
I blame S.M. Stirling.
PS I'm also filing this stuff away under "potential science fair projects for future offspring."