Sand has a peculiar physical property known as hysteresis. The
dictionary defines hysteresis as a condition in which a reaction is delayed or
lags its cause. In sand, the reaction is not only delayed, it is absorbed and
Part of the reason it hurts so much to crash into a brick building has to do with Newton's Third Law of Motion, "For every action there is an equal an opposite reaction." You hit the bricks, the bricks remain almost stationary, and the force of the impact is painfully absorbed by your body. Hit some sand, however, and the force moves the sand. There is a reaction; the laws of physics are not violated. But the energy is dispersed throughout the sand particles. The amount of energy that your body must absorb is drastically reduced.
The Wrights were well aware of the dangers of exploring manned flight; many investigators had been killed or injured before them. They were also aware that sand's unique properties offered a kind of "safety net" for people falling out of the sky. So they insisted on a sandy location to learn to fly. And a good thing, too, for their own records show they crashed many times.
Sand and Hysteresis