Print some accessories to to attach to a rat trap and turn it into a spring-loaded car! Note that I have not yet successfully gotten this to work. Much more detailed information is available on this blog post: http://netninja.com/2013/11/03/on-designing-and-building-the-rat-racer Download ratracer-src.zip for the OpenSCAD source for all three components.Pull back the U-bar and, while carefully holding it down and not releasing it onto your fingers, thread the string through the hole in the axle. Wind it around the axle. Put the car on the floor. Quickly release the snappy part. In theory, the string is pulled, spinning the axle and wheels, making the car go. In practice, I haven't gotten that far yet.