  Here I discuss some implicit assumptions of modern hydrodynamic models and
argue that their accuracy cannot be better than 10-15 %. Then I formulate the
correct conservation laws for the fluid emitting particles from an arbitrary
freeze-out (FO) hypersurface (HS) and show that the derived momentum
distribution function of emitted particles does not contain negative
contributions which appear in the famous Cooper-Frye formula. Further I analyze
the typical pitfalls of some hydro models trying to alternatively resolve the
FO problem.
