The idea is to pull one metal sphere away from the others and release. It bangs into the other four and out the other side flies one sphere. Pull two spheres out and release and you'll see two spheres fly out the other side. Pull three out and you'll see three fly. Same goes for four. Why? Conservation of momentum and energy. If you have one sphere's worth of energy going into the collision, you must also have one sphere's worth of energy going out. Same goes for two, three, and four spheres.
Recently I took my 8 year old to a science demonstration show and they brought out a scaled up Newton's Cradle using bowling balls! Very cool!!!
Newton's Cradle can also be soothing to watch and a could be time killer if you're really bored. They're also very cheap to purchase. A quick search on Amazon.com reveals Newton's Cradles for as cheap as $12.