[herodotus]

The invention that impresses me is the differential gear. The National Museum of Scotland, when I lived there, had a cut-away exhibit taken from a car. I played with it, and for the first time, understood how it worked. Two days later, I could not reconstruct it in my mind! Unclear who invented it.

Here is a link to the Wikipedia entry: https://en.wikipedia.org/wiki/Differential_(mechanical_devic...

Mind you, this may not be a good answer to the question, because there are a couple of claims for the invention.

[evanb]

There's a great old-timey video explaining the differential in a car works: https://www.youtube.com/watch?v=yYAw79386WI

[petepete]

Wasn't it invented by the ancient Greeks?

https://en.wikipedia.org/wiki/Antikythera_mechanism