The Pinewood Derby is a traditional crafting event among Cub Scouts in the United States. Boys are given a block of wood and some plastic wheels. Their job is to carve and paint the block into a car that, when released at the top of a long ramp, will beat the other similarly made cars to the finish line. When I was a Cub Scout, my Dad showed me how to add weights to the body. Doing so was completely within the rules. Adding CO2 canisters to the car, however, probably isn't. It's actually a fairly sophisticated modification because the tubes have to open at the right time. A video at the link shows how it's done.
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