  In a Newtonian system with localized interactions the whole set of particles
is naturally decomposed into dynamical clusters, defined as finite groups of
particles having an influence on each other's trajectory during a given
interval of time. For an ideal gas with short-range intermolecular force, we
provide a description of the cluster size distribution in terms of the reduced
Boltzmann density. In the simplified context of Maxwell molecules, we show that
a macroscopic fraction of the gas forms a giant component in finite kinetic
time. The critical index of this phase transition is in agreement with previous
numerical results on the elastic billiard.
