  A simple model of how objects of different masses stream towards each other
as they cluster gravitationally is described. The model shows how the mean
streaming velocity of dark matter particles is related to the motions of the
parent dark matter haloes. It also provides a reasonably accurate description
of how the pairwise velocity dispersion of dark matter particles differs from
that of the parent haloes. The analysis is then extended to describe the
streaming motions of galaxies. This shows explicitly that the streaming motions
measured in a given galaxy sample depend on how the sample was selected, and
shows how to account for this dependence on sample selection. In addition,we
show that the pairwise dispersion should also depend on sample type. Our model
predicts that, on small scales, redshift space distortions should affect red
galaxies more strongly than blue.
