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 tenticles. Our everyday activities turn on the performance of nature's main engine: we may breathe harder going uphill but we put more strain on our muscles walking downhill. Those of us who are right-handed can tighten screws and lids more forcibly than we can loosen them. This text discusses how form and performance make these things happen - illustrating nature at work.