Excellent educational scientific tool not only shows the pendulum but also shows the law of conservation of momentum and energy.
Pull back one or more of the balance balls, release them and the same number of balls will swing up on the opposite side.
The science behind it:
The Newton's cradle is a device that demonstrates the conservation of momentum and the conservation of energy with swinging spheres. When one sphere at the end is lifted and released, it strikes the stationary spheres, transmitting a force through the stationary spheres that pushes the last sphere upward. The last sphere swings back and strikes the nearly stationary spheres, repeating the effect in the opposite direction. The device is named after 17th-century English scientist Sir Isaac Newton.