Users can create, shape and manipulate anything that is gaseous, a phase of matter characterized by relatively low density, high fluidity/no definite shape, and lack of rigidity. Gas is very compressible but tends to expand indefinitely, and it fills any container. A small change in temperature or pressure produces a substantial change in its volume. As most gases are difficult to observe directly, they are described through the use of four physical properties or macroscopic characteristics: pressure, volume, number of particles (chemists group them by moles) and temperature.