  Time evolution of a "little bang" created in heavy ion collisions can be
divided into two phases, the pre-equilibrium and hydrodynamic. At what moment
the evolution becomes hydrodynamic and is there any universality in the
hydrodynamic flow? To answer these questions we briefly discuss various
versions of hydrodynamics and their applicability conditions. In particular, we
elaborate on the idea of "universal" (all-order resumed) hydrodynamics and
propose a simple new model for it. The model is motivated by results obtained
recently via the AdS/CFT correspondence. Finally, charged hadron multiplicities
in heavy ion collisions at the RHIC and LHC are discussed. At the freezout, the
multiplicities can be related to total entropy produced in the collision.
Assuming the universal hydrodynamics to hold, we calculate the entropy
production in the hydro stage of the collision. We end up speculating about a
connection between the multiplicity growth and the temperature dependence of
the QGP viscosity.
