This great old video is a remarkably clear explanation of how a differential gear works -- the kind of thing that you could show to a young child or an adult and make the lightbulb go on for both of them.
I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.