  We have carried out a hydrodynamical code comparison study of interacting
multiphase fluids. The two commonly used techniques of grid and smoothed
particle hydrodynamics (SPH) show striking differences in their ability to
model processes that are fundamentally important across many areas of
astrophysics. Whilst Eulerian grid based methods are able to resolve and treat
important dynamical instabilities, such as Kelvin-Helmholtz or Rayleigh-Taylor,
these processes are poorly or not at all resolved by existing SPH techniques.
We show that the reason for this is that SPH, at least in its standard
implementation, introduces spurious pressure forces on particles in regions
where there are steep density gradients. This results in a boundary gap of the
size of the SPH smoothing kernel over which information is not transferred.
