Hydrophobic sand (or magic sand) is a toy made from sand coated with an 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.
History
The earliest reference to waterproof sand is in the 1915 book The Boy Mechanic Book 2 published by Popular Mechanics. The Boy Mechanic states waterproof sand was invented by East Indian magicians. The sand was made by mixing heated sand with melted wax. The wax would repel water when the sand was exposed to water. Magic sand was originally developed to trap ocean oil spills near the shore. This was done by sprinkling magic sand on floating petroleum, which would then mix with the oil and make it heavy enough to sink. Due to the expense of production, however...