No kidding, I built one at work for a coworkers son a few years back and he got an A and the teach kept it for an example for future classes. I made axles to put on the bottom of the car and the rear axle had a pulley in the center with string wrapped around it, the other end of the string was tied to the arm on the mouse trap, and when the trap was triggered it pulled the string and off it went. He had to come up with some good wheels though, because the wheels would just spin other wise, I think he put some wheels from a model car that had rubber tires on it (a plastic model kit wheels) also have to put a hole through the trap for the string to go down to the pulley on the axle. Hope this helps you out.
Dauntae