Physically, the three states of matter (solid, liquid, and gas) can be distinguished by their different behavior in response to changes in temperature and pressure. Solid materials tend to remain rigid and their shape is fixed, while liquids flow and conform to the shape of their containers, and gases expand freely to fill any container. Mathematically, the three states of matter can be distinguished by their different characteristic equations of state. The equation of state for a solid relates the pressure, volume, and temperature of the material, while the equation of state for a liquid relates the pressure, temperature, and surface tension of the material. The equation of state for a gas relates the pressure, temperature, and volume of the material.