The sperm’s tail that whips around to make the sperm squirm toward the egg looks like a tail, except that it is built out of an assemblage of proteins. The motor that makes the tail spin resembles a motor, with a set of moving parts arranged in a circle. And the hook that connects the motor to the tail, transforming circular motion to the propeller-like, swimming motion of the sperm, looks like a hook engineered precisely to achieve this transformation.
