  We consider the classical Kac's model for the approximation of the Boltzmann
equation, and study the correlation error measuring the defect of propagation
of chaos in the mean field limit. This contribution is inspired by a recent
paper of the same authors where a large class of models, including quantum
systems, are considered. Here we outline the main ideas in the context of grand
canonical measures, for which both the evolution equations and the proof
simplify.
