  In this paper we present properties of relativistic and non-relativistic
perfect hydrodynamical models. In particular we show illustrations of the fact
that different initial conditions and equations of state can lead to the same
hadronic final state. This means that alone from the hadronic observables one
cannot determine either of the above, one needs for example penetrating probes
that inherit their properties from each timeslice of the evolution of the
fireball.
