Let's put a car at the top of the track, and watch it click and clack its way down the four colorful tracks. We can do one car at a time, or set them off one right in front of the other, or set one off the moment the other finishes so the action never stops.

We can start our cars off whenver we like, and in any combination. We can start one halfway down and another at the top at the same time. Or leave one at the bottom and see if the other car pushes it off the track. We can hold one back at the top until we say "GO."