By balancing on just two supports, we exist in permanent defiance of gravity. As toddlers amusingly demonstrate, walking is essentially a matter of hurling the body forward and letting the legs run to catch up. A pedestrian in motion has one foot or the other off the ground for as much as 90 percent of the time, and thus engages in constant unconscious adjustments of balance. In addition, our center of gravity is high—just above our waists—which adds to our innate tippiness.
