Dispersion is the fact that one substance is in small pieces included in another. Liquid or solid products can be dispersed in liquid or gaseous vehicles and vice versa. Dispersions often used as a dosage form because they allow the administration of products that are not soluble in water. A dispersion is a system in which discrete particles of a material are continuously dispersed in the phase of another material. The two phases can be identical or different states of matter. They are different from solutions, where the dissolved molecules do not form a separate phase from the solute. Dispersions are categorized in different ways, including particle size as they relate to the particles in the continuous phase, whether or not precipitation occurs, and the presence of Brownian motion. Generally, dispersions of particles large enough for sedimentation are called suspensions, while those of smaller particles are called colloids. It is still a common belief that dispersions have no structure; that is, the particles dispersed in the liquid or solid matrix are assumed to be statistically distributed.