Another way of distinguishing between systems is their classification into homogeneous and heterogeneous systems. A homogeneous system has the same composition and properties throughout. Such systems are said to comprise only one phase, and are thus termed monophasic. In contrast, heterogeneous systems are composed of more than one phase. The concept of homogeneity is related to the scale on which we consider the material of the system, and this must always be defined. For example, milk that is homogeneous on a macroscopic scale consists of a heterogeneous colloidal suspension of fat droplets and proteins when examined at the micrometer level. Similarly, a clear “homogeneous” gel consists, at the scale of tens or hundreds of nanometers, of two distinct phases: water and hydrated polysaccharides.