Hydrophobic sand (or magic sand) is a toy made from sand coated with a hydrophobic compound. The presence of the hydrophobic compound causes the grains of sand to adhere to one another and form cylinders (to minimize surface area) when exposed to water, and form a pocket of air around the sand.  The pocket of air makes magic sand unable to get wet.  A variation of this, kinetic sand, has several of the same properties, but acts like wet sand that will not dry out.  Hydrophobic sand, whether the wet or dry type, will not mix with water.