An exact equation describing the stochastic motion of a droplet of nei
ghbouring equal mass hard points with velocities +/- c, immerzed in a
hard point fluid, is derived, The derivation generalizes Resibois' ide
a originally applied to the study of self-diffusion. The explicit solu
tion for a two-particle droplet is obtained, showing a rapid relaxatio
n of the interparticle distance to the Poisson distribution, superpose
d on a slow diffusive motion of the centre of mass of the two particle
s. An approximate description of the dynamics of N-particle droplets (
N > 2) is proposed.