How a ramp walking toy works.
Click on the image above to download a PDF copy of the passive dynamic walker.
There is some serious science behind the physics of it.
A study of simple unactuated bipeds by Meghan Murray.
The study of passive dynamics walkers was pioneered by Tad McGreer in the 1980s followed by Andy Ruina and Art Kuo.
Knees, according to McGeer, solve the problem of feet colliding with the ground when the leg swings forward.
The record for distance travelled by walking robots is 65.17 km, held by the passive dynamics based Cornell Ranger.