From a physical point of view a fluid is a material that lacks shear strength, i.e. that deforms, or “flows”, when subject to shear stress. The strain rate for a given shear stress is of course highly variable, and defines the viscosity of the fluid. From a chemical thermodynamic point of view it is convenient to distinguish between different types of fluids. There are fluids with relatively low densities and low viscosities, which tend to be highly mobile in planetary environments. These are often referred to as “volatiles”, and are typically composed of species in the system C–O–H–N–S–F–Cl, with inert gases (particularly He) also important in gas giants. There are also fluids with generally higher densities than volatiles, which also have much higher viscosities, typically by several orders of magnitude. If such fluids exist at equilibrium with solids of broadly similar bulk composition we call them melts (Chapter 10). Melts in terrestrial planets are chiefly silicates, although natural carbonate melts also exist, as do metallic melts in planetary cores. Melts in icy satellites, in contrast, are likely to be composed chiefly of species in the system C–O–H–N. A third type of fluids are liquids at conditions that are far removed from equilibrium with solids of similar bulk composition, but that may contain species in solution that crystallize their own solids.