Bicycle
I wrote my master thesis on optimizing bicycle wheels / spokes. I actually see I'm cited in the phd he cites, quite a fun surprise!This is a great article. It showcases lots of the "simple, but surprisingly advanced" things surrounding bicycles. Which was what got me hooked in the first place. The visualization of how you have to turn right to go left is excellent. I've mentioned that fact multiple times here on HN, it's not commonly known, you just "do it" when you bike! And it explains why you sometimes can feel the curb "sucking" you towards it when you try to avoid it: you unconsciously avoid turning the wheel towards it, but that actually makes it so that you're unable to actually steer away from it!