Part 1 features the logical progression of engineering mechanisms developed to accomplish The Cycle of Operation:

1. chambering
2. locking
3. firing
4. unlocking
5. extraction
6. ejection
7. cocking
8. feeding

Part 2 and Part 3 build upon this basic model, ending with a fully functional firearm.
If you liked these videos, check out this other one about differential steering.
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Skip to about 2 minutes to avoid the intro.
A commenter recommended Feynman explaining the alternative to a differential drive that trains employ.
I’ve been forgetting “via”s, which I suppose isn’t fair: [via reddit]