Rheology is science of flow, of deformation of liquids and solids, and of the in-between viscoelastic materials, such as asphalt, glass, dough or plastics (polymers). The properties of the materials are described by the constitutive equation (rheological equation of state) which may an include other then mechanical quantities, such as magnetic or electrical fields. Constitutive equation describes the memory of materials, effects of thermal and stress history and effects such as hysteresis.