New Scientist magazine writers have built a robot that may date back as far as 150 B.C. Using a system in which string is wrapped around an axle and pulled by a weight, this “robot” is actually a cart that can move on its own, using only mechanical power.

As the weight falls towards the ground, the string is slowly unwound from the axle, thus turning the axle and the wheels. If the string is wound one way, the robot will move forwards, and if you wind it the other way, it goes backwards. You can place pins at different points along the axle and wind the string one way, then around the pin, then the other way— in effect, programming the robot to move forward, then stop, then reverse direction.

This robot was built from scooter wheels and broomsticks. The distance it can travel is limited only by the length of the string and how far the weight can fall.