We have a model of an ideal gas which has ball bearings in a clear Perspex tube which are agitated when a motor hits a rubber floor. The ball bearings then hit a polystyrene block so hard that they make it levitate. Pressure is caused by the rate of change of momentum of the balls over unit area. In this picture, the ball bearings are not visible because they were moving so fast. The internal energy of a substance is the sum of the kinetic and potential energies of the particles. For a solid, bond energies contribute to the potential energy. It is much simpler in a gas as there are no potential energies to take into account, so the internal energy is the sum of the kinetic energies of the gas particles.
