Kottke reminded me this morning about this mid-century video that explains, very clearly, how a differential gear works in automobiles. Watch:
Email readers can watch here.
Notice how the video starts with a scene that everyone can grasp before going into more technical information. They focus on "why" before "how". The simple model they use to explain the mechanism is a great example of simplification. The "noise" of the actual gear is removed so the instructor can focus on the big ideas. A truly remarkable explanation.