Sand art is the practice of modelling sand into an artistic form, such as a sand sculpture, sandpainting, or sand bottles. A sand castle is a type of sand sculpture resembling a miniature building, often a castle.
The two basic building ingredients, sand and water, are available in abundance on a sandy beach, so most sand play takes place there, or in a sandpit. Tidal beaches generally have sand that limits height and structure because of the shape of the sand grains. Good sand sculpture sand is somewhat dirty, having silt and clay that helps lock the irregular shaped sand grains together.
Sand castles are typically made by children, simply for the fun of it, but there are also sand sculpture contests for adults that involve large, complex constructions.
An example of extremely sophisticated sand art is the Buddhist mandala.
Sand grains will not stick together unless the sand is reasonably fine. While dry sand is loose, wet sand is adherent if the proper amounts of sand and water are used in the mixture. The reason for this is that water forms little ‘bridges’ between the grains of sand when it is damp due to the forces of surface tension. However if too much water is added the water fills the spaces between the grains, breaking down the bridges and thus lowering the surface tension, resulting in the sand being able to flow more easily and the structure collapsing. Some believe the ideal ratio is eight parts dry sand to one part water, though this may depend on the type of sand.