It took me quite a while to understand exactly what was happening with a Newtons Cradle toy. You know the one with the clacky balls that swing...<p>If you hold 2 balls up, and let them fall, 2 balls swing up on the other side. How does it know how many you swung??? It boggled my mind for the longest time. Then I got to playing with one, and found something interesting. It doesn't always seem to work!<p>If the balls are perfectly aligned, and touching each other, the effect is dampened by the multi-body collision. It's strongest when the balls are just <i>barely</i> not touching.<p>What we see as multiple balls swinging and hitting is really a whole bunch of individual 2 body collisions that are close enough in time to seem like they are a single event.<p>I could be wrong about the reason, but this was my observation.