Build a Mouse Trap Car: Backwards Motion Help

[Kerri23]

I am suppose to build a mouse trap car ...it must go 2m forward, pick up an object and go 2m backwards without me touching it. i can get it to go forwards and pick up the object but not backwards. my design is fairly simple..there's a string attached to the axle.. the other end is attached to the part that comes down on the mouse. you pull that part back and wind up the string by turning the axle back. when the part that hits the mouse springs forward, it unravels the string and spins the wheels. but i have no idea how to make it go backwards. ( it goes backwards if you wind it up the other way though)

any hot ideas would be appreciated.
-note: i can only use one mouse trap

 

[Danger]

Welcome to PF, Kerri.
Have you considered devoting all of the mechanical bits to forward and backward propulsion, and simply Velcroing the 'mouse' and the front of your vehicle?
Keep in mind here that I think farther outside of the box than most. Rules can't be broken, but I like to bend them into the form of singularities.