I've added one new wrinkle that we've not done before. We're going to randomly select races (two in each den group) and when those heats are up, we have a red light that's going to go off. Within that race we will randomly select a place (1st through 4th). The boy who made the car that finishes in that place will get to draw a prize from a grab bag. They're all Hot Wheel cars.
It's another way to try to keep kids excited and invested in the races, since they'll never know when the race prize light will go off. Or do they? 
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A long time ago I read about a paradox of the "Unexpected Egg" which has been rewritten a number of different ways. It has to do with events and scheduling. Using the random race prize example above the reasoning would go like this:
1. There can never be an unexpected race. If we know there are 10 races and 1 prize, then the prize can never be on the 10th race because all of the other races would have been run already so we know that the prize race is the 10th. By that reasoning it cannot be the 10th race.
2. Now that we've eliminated the 10th race, we know it can't be the 9th. Once we get to the 9th race we know it has to be a prize race because the 10th race cannot be one, due to the reasoning in point 1.
3. By applying the same reasoning, we can eliminate all races as possible random prize races, therefore they can never occur.
4. Yet, when the 5th race comes along and the light goes off, we're still surprised!
Something like that, anyway. If you're in to this sort of thing, read up on the paradox of the Unexpected Egg.
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