A liquid is one of the main phases of matter. It is a fluid whose shape is usually determined by the container it fills. Liquid particles (normally molecules or clusters of molecules) are free to move within the liquid volume, but their mutual attraction limits the ability of particles to leave the volume. The volume of a quantity of liquid is fixed by its temperature and pressure. Unless this volume exactly matches the volume of the container, a surface is observed. The surface of the liquid behaves as an elastic membrane in which surface tension appears, allowing the formation of drops and bubbles. Capillarity is another consequence of surface tension. Liquids are generally resistant to compression: water, for example, does not change its density appreciably unless subject to pressure of the order of a gigapascal.