engine design

A 1937 GM-produced film explaining how the automobile differential lets wheels turn at different speeds around a corner. It uses clear animations and cutaway demonstrations to build the idea from first principles.

A short clip of Richard Feynman explaining how a train stays on its tracks, through the conical shape of wheels and how it handles curves without differentials.

Ward Cunningham explains the history and intent of the debt metaphor, which he coined while refactoring the WyCash product. He says debt means code reflecting an incomplete understanding of the problem, not sloppy programming.