  The regularity of solutions to the Boltzmann equation is a fundamental
problem in the kinetic theory. In this paper, the case with angular cut-off is
investigated. It is shown that the macroscopic parts of solutions to the
Boltzmann equation, i.e. the density, momentum and total energy are continuous
functions of $(x,t)$ in the region $\mathbb{R}^3\times(0,+\infty)$. More
precisely, these macroscopic quantities immediately become continuous in any
positive time even though they are initially discontinuous and the
discontinuities of solutions propagate only in the microscopic level. It should
be noted that such kind of phenomenon can not happen for the compressible
Navier-Stokes equations in which the initial discontinuities of the density
never vanish in any finite time, see \cite{Hoff}. This hints that the Boltzmann
equation has better regularity effect in the macroscopic level than
compressible Navier-Stokes equations.
