The story-and the science-of nature's greatest engine. Whether we blink an eye, lift a finger, throw a spear or a ball, walk, run, or merely breathe, we are using muscle. Although muscles differ little in appearance and performance across the animal kingdom, they accomplish tasks as diverse as making flies fly, rattlesnakes rattle, and squid shoot their tentacles. Our everyday activities turn on the performance of nature's main engine: we may breathe harder going uphill, but we put more strain