Any child who plays on the beach knows that the physical properties of dry and wet sand are quite different: it is impossible to build a castle with only dry sand. Sand grains are subjected to three types of forces: gravity, the contact force between the grains, and electrostatic forces.
Due to the size of the sand grains, weight – that is, gravity – predominates over electrostatic forces, so that they tend to descend and squeeze. This is so in the dry state. When a small amount of water is poured onto the sand, the grains are enveloped by a very thin water film. It should not be forgotten that the water molecule is electrically polar, that is, it has an excess of negative charge in one area and a predominance of positive charge in the opposite area. In this way, the water molecules close to the wall of the charged sand grain are attracted and stick together . Thus, it is the water retained between the walls of two neighboring grains that acts as an effective natural cement .
