To a geologist, sand is any collection of grains of rock between 0.02 and 2mm in diameter. It is formed by the erosion of rocks in several different environments.

On beaches and in rivers, for example, the erosion is caused by the movement of the water.

Desert sand is produced by wind blowing small rock particles against each other so that they become ground down into smaller and smaller pieces. Geologists can tell how the grains in sandstone were formed by looking at the distinctive shapes of the grains. Soil is formed in similar ways to sand by the erosion of rocks, but there is an important difference. Soil contains organic matter &endash; the remains of plants and animals. This organic matter forms humus, which mixes with rock particles and allows plants to grow in the soil.