KSprot Newtons Cradle Balance Balls Physics Pendulum Science Desk Office Classic Toy (Black) Newton's Cradle makes a great desktop gift. These swinging balls will keep you occupied for hours at work. Material: Plastic + Metal balls + Fishing Line Size: 16 * 11.7 * 8.5CM Newton's Cradle is a clever device that not only demonstrates a pendulum, but also shows the Laws of Conservation of Momentum and Energy.