  We investigate using numerical simulations the domain of applicability of the
hydrodynamic description of classical fluids at and near equilibrium. We find
this to be independent of the degree of many-body correlations in the system;
the range r_c of the microscopic interactions completely determines the maximum
wavenumber k_{max} at which the hydrodynamic description is applicable by
k_{max}r_c ~ 0.43. For the important special case of the Coulomb potential of
infinite range, we show that the ordinary hydrodynamic description is never
valid.
