Almost all known substances undergo
thermal expansion in response to heating, meaning that a given mass of substance contracts to a low volume at low
temperatures, when little
thermal energy is present. Substances, especially
fluids in which
intermolecular forces are weak, also undergo
compression upon the application of
pressure. Nearly all substances therefore reach a density maximum at very low temperatures and very high pressures, characteristic properties of the
solid state of
matter. ==Water==