Interatomic forces act like miniature springs connecting the atoms.
As a result the whole system oscillates like a bunch of harmonic oscillators. In this section we demonstrate the above assertions by making a crude model of the quantum mechanical states of a brick. This assumption is a rather poor approximation to the behavior of a solid body when the total amount of internal energy is so small that many of the harmonic oscillators are in their ground state. However, it is adequate for situations in which the energy per oscillator is several times the ground state oscillator energy. We further assume that each oscillator is weakly coupled to its neighbor.