I was going to write something else before the weekend, but I just stumbled on this absolutely genius instructional video on how differential gears work. I've known the point of a differential - give the same force to two wheels, even though they rotate at different speeds - but I've never quite wrapped my head around exactly why it works. This video made the trick.
I love almost everything about it - the black and white tones and the lighting is excellent; the examples and explanations are exemplary, though exceedingly eloquent. The one slightly annoying thing is the narrators cadence, which is just a little over. the. top. as. he. empha. sizes. every. single. word.
The video doesn't bring it up - it's about automotive engineering - and I'm not going to belabour it, but if you control the center piece instead of letting it rotate freely, you can use it to set the relative speed and the phase of rotation between the two output axes. Me and a colleague had a vague idea for using that to create a mechanically simple four-legged walker, but as so often we never found the time and motivation to actually do anything with it.