The car works when one end of a string is tied to the arm on the mousetrap and the other end is wound around an axle. By winding the string around the axle the mousetrap's spring is stretched providing stored energy. As the mousetrap is released it pulls the string off of the axle causing the wheels to turn and making the car move.
There are two Mousetrap Car Projects. Just click on the link of each project to visit the webpages: