Grain size refers to the average diameter of individual grains in a material, such as soil, rock, or metal. It is an important factor in determining the physical properties of these materials, including strength, porosity, and permeability. Smaller grains typically result in denser and stronger materials, while larger grains can lead to more porous structures. In geology, grain size is often classified into categories like clay, silt, sand, and gravel. This classification helps scientists understand the formation processes of sedimentary rocks and the behavior of soil in various environments. Grain size can also influence how materials interact with water and other substances.