A Newton's cradle (here in LEGO by Monsterbrick) is a device that demonstrates the principle of the conservation of momentum. Five steel balls are suspended from a frame so that they are just touching. As the ball on the left is lifted up and let go, it swings towards the other balls. When it strikes them it stops, but that momentum has to go somewhere. It is transferred through the balls to the right hand ball, which swings out. As this reaches the top of its arch it swings back, repeating the pattern. If there were no loss of energy due to friction this could go back and forth forever.