  One of the questions in the cosmology courses is the cooling mechanism of
cosmic fluid during it expansion according to classical concepts of the
thermodynamics. In this short pedagogical paper, we quote the questions and
give a natural approach dealing with this problem by measuring the dispersion
velocity of the particles in the cosmic fluid by a comoving observer. We show
that the thermal motion of the particles in the cosmic fluid deviates from the
Hubble flow and follows the geodesics governed by the gravity of the
homogeneous universe. The dynamics of the "thermal peculiar velocity" of the
particles leads in an expanding universe, momentum of the particles relax by
the inverse of the expansion factor and the result is losing the energy of
particles, hence cooling the universe.
