We make comments on a paper by Parravicini and Rigamonti (2011 Eur. J. Phys. 32 169). It seems to us that the macroscopic explanation of thermalization using the continuum mechanic approach is not enough. As matter is presumed to be composed of many particles, here, we offer a microscopic explanation by considering the collisions or scattering of many particles relevant to the thermalization process.