The kinetic molecular theory of Maxwell and Boltzmann proposes that the macroscopic properties of an ideal  gas  derive  from the fact that it is  composed of a large number of colliding point particles that  are  in rapid motion.

The particles do not interact with one another except in elastic collisions. The piston receives from each such collision a certain amount of momentum. The macroscopic effect of these collisions is pressure.

Click and drag the piston horizontally to modify the volume,