Newton's Cradle Art in Motion, 7 1/4-Inch Balance Balls - Black Wooden Base Plastic Imported The classic desk accessory is art-in-motion, Newton's Principle that "action and reaction are equal and opposite." Use it to waste time, relieve stress, tune out boring coworkers, or schooling youngsters in physics. Lift one of the 5 steel balls at either end and let it swing back. Fair warning: it's totally addictive! 7.25".