  The mutual influence of two layers with strongly loclized electrons is
exercised through the random Coulomb shifts of site energies in one layer
caused by electron hops in the other layer. We trace how these shifts give rise
to a voltage drop in the passive layer, when a current is passed through the
active layer. We find that the microscopic origin of drag lies in the time
correlations of the occupation numbers of the sites involved in a hop. These
correlations are neglected within the conventional Miller-Abrahams scheme for
calculating the hopping resistance.
