We've been looking at the Frugal Fun for Boys blog, and Seth got the idea to make this from one of their posts. I'm going to let him explain it in his own words. Take it away, Seth!
"First, you nail four nails on a square piece of wood, one in each corner."
"Second, you put four rubber bands onto the nails."
"Third, you pull the car back against the rubber bands."
"Then you let it go. So the idea is you pull two cars back and they bounce against the rubber bands, and the one that hits the other car is the winner."