i am suppose to build a mouse trap car for school...it must go 2m forward, pick up an object and go 2m backwards without 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. any hot ideas would be appreciated.