  We discuss the evolution of linear perturbations about a
Friedmann-Robertson-Walker background metric, using only the local conservation
of energy-momentum. We show that on sufficiently large scales the curvature
perturbation on spatial hypersurfaces of uniform-density is constant when the
non-adiabatic pressure perturbation is negligible. We clarify the conditions
under which super-horizon curvature perturbations may vary, using preheating as
an example.
