While warming up for a lunchtime basketball game, engineer Howard Stone and applied mathematician Jeff Aristoff wondered whether you could model the shape of a jump rope. They gave it a spin. They considered the centrifugal force, tension, air drag, made a mini robot jump-roper, took high-speed video of a colleague jumping rope, and developed a detailed mathematical model that describes the shape of the rope in the air. If nothing else, it was a good exercise, they say.