Just a Spoonful of Sand?

What is it about sand that makes kids want to eat it? And eat it by the handful (or shovelful in Allie's case)? It can't possibly feel good in their mouth or taste good. I wonder what kind of mineral they gain from eating sand that they're obviously lacking in their diets. Must be something only kids need because I have no desire to go digging through Allie's sandbox for a nice spoonful of sand. Hmmm, one of the great mysteries of parenthood. (Maybe they do it just to keep us guessing....)