This is a 3D printed mouse trap racer, which demonstrates the physics principle of the conversion of potential energy to kinetic energy by using the spring of a mouse trap to spin the axle of the wheels.

EDIT: By popular request I recorded a quick video of the racer in action. http://www.youtube.com/watch?v=dZ3QAxe7-LE

In order to form the wheels you have to either super glue them together or use acetone to melt them together.