Carbon nanotube yarn could help create battery-free wearables, powered entirely by their users' movement. Imagine being able to harness the energy produced during your morning jog and using it to power a music player or fitness tracker. Researchers have developed a special ultra-thin yarn created from carbon nanotubes. It efficiently converts mechanical energy into electrical energy. The yarn can be twisted into elastic-like coils that allow the threads to generate electricity when stretched. The energy from one piece of yarn can generate 250 watts per kilogram when a number of them are bound together and stretched 30 times per second.