The boys soon realize that if they want to build a sand castle they must use it wet … in its proper measure. If they only use dry sand, the most they get is a scattered mound, but if they carry too much water with their buckets, the building collapses, like when a wave hits . Who is behind everything is an old friend of ours: electricity.
When the sand is wetted, the water covers the surface of the grains with a thin layer. Water is an extraordinary substance and among its properties is that the molecule has its electrical charges poorly distributed. In short, that one part of the molecule is positively charged and the opposite, negatively. Not much, but enough for chemists to say that water is a polar molecule. Something that comes in handy because water dissolves a much greater number of substances than any other ; It is not for nothing that it is known as the universal solvent, which came in handy in the appearance of living matter.
For their part, the grains of sand also present curious distributions of electrical charges, which means that when a molecule of water approaches the grain of sand it stays stuck in the same way as if we played to attract pieces of paper with a ballpoint pen rubbed in the sleeve of the sweater. In this way, the water trapped between the surface of two grains acts as a weak but effective electric glue. But if there is too much water, the grains separate and it will be the end of the sand castle.
