  Motivated by the large effect of turbulent drag reduction by minute
concentrations of polymers we study the effects of minor viscosity contrasts on
the stability of hydrodynamic flows. The key player is a localized region where
the energy of fluctuations is produced by interactions with the mean flow (the
"critical layer"). We show that a layer of weakly space-dependent viscosity
placed near the critical layer can have very large stabilizing effect on
hydrodynamic fluctuations, retarding significantly the onset of turbulence. The
effect is not due to a modified dissipation (as is assumed in theories of drag
reduction), but due to reduced energy intake from the mean flow to the
fluctuations. We propose that similar physics act in turbulent drag reduction.
