Newton’s Cradle
Lift one ball and release it. When it strikes the others, a ball at the opposite end swings out.
During the collision, momentum is transferred through the row of balls. The total momentum of the system is conserved. Because the collisions are also nearly elastic, most of the kinetic energy is transferred as well.
Energy is lost gradually to air resistance and sound each swing, which is why the balls eventually settle down instead of swinging forever.
What happens if you release one, two, or three balls at once?
Additional Information
This demonstrates both conservation of momentum and conservation of energy at the same time.
Lift and release two balls together, and exactly two balls fly off the other end, never one ball moving twice as far.
The middle balls barely move at all — they pass the momentum along almost instantly, like a shockwave through the row.