The first day of a robotics class, my teacher explained that we don't really walk. Each step is graceful falling. Place one foot forward, tilt and fall till the foot hits and repeat.
If walking was simply graceful falling, it would imply that once we begin the process of "falling" onto the next foot, we couldn't significantly affect the outcome. However, because I can start walking and then stop with a single leg half way between it's highest point and the ground, this explanation of walking as falling becomes very problematic.