Newton's cradle is a toy.

Isaac Newton did not invent it, nor did he invent the Laws of Motion. They just are. He uncovered what always, as far as we know, existed.

If you use this in class, do not show the kids the various permutations--they will find them if you let them be. Do not tell them it models the Law of Conservation of Momentum.

And if they ask for an explanation, tell them that everything moving (which is everything) has a certain amount of oomph, depending on how much stuff it has, and which direction it's moving. If they ask for more, tell them that we have just so much oomph in the world, no less, no more, and that it can be passed along between things.

If they ask why, tell them no one knows why. If you tell them otherwise, you will confuse them. Mutatio motus just is.

Just let them play, touching and seeing and hearing the world as it is.

You can play with a computerized version here, using different numbers of balls. 

But why not just use the real thing?
Yes, I know Newton was reporting what others had already shown.... 

The cradle pictured is by Dominique Toussaint from Wikimedia.