Newton's cradle. This device consists of a set of balls resting against one another that are free to swing from the bars they hang from. Two of the balls (centre right) have been sent swinging. The kinetic energy with which they strike the stationary balls will be transferred from ball to ball, finally propelling the furthest two balls away from the others. These balls will in turn swing outwards until gravity pulls them back down, striking the stationary balls again. This process continues to occur until energy is lost to friction bringing them to rest.