Quicksand is a soft mass of wet sand that yields easily to pressure. It usually forms on the bottoms of streams and on sand flats along seacoasts. Quicksand behaves like a fluid because water flowing through the sand grains forces them apart and prevents them from settling. In this quick condition, the sand loses its firmness and cannot support heavy weight. Thick layers of quicksand can trap and even kill people or animals that sink into them.