It's one of the simplest energy-storage devices known to man: The spring. Think of how a jack-in-the-box keeps hold of the mechanical energy it takes to compress that clown into the box, releasing it only when the weasel song reaches its climax. And that energy storage is a long-term proposition. The clown could likely sit, poised in that box in grandma's attic for 100 years, until some joker comes along, cranks the handle and, POP! Now imagine millions of carbon nanotubes -- tube-shaped molecules of pure carbon -- all storing as much energy, pound-for-pound as a comparable lithium-ion battery, then releasing that energy to give power to a lunar rover, a silent leaf blower or even a car.