Our bodies contain myriad motors to produce mechanical work. We're only too aware of the muscles that propel our limbs or pump our hearts; less noticed are the minuscule molecular motors at the cellular level that accomplish such chores as transporting organelles. All these actions involve motor proteins, enzymes that convert the chemical energy stored in the molecule adenosine triphosphate—known as ATP—into mechanical energy. Exactly how they channel the energy into rectified motion remains a compelling mystery.