  Some basic questions about the hydrodynamical approach to relativistic heavy
ion collisions are discussed aiming to clarify how far we can go with such an
approach to extract useful information on the properties and dynamics of the
QCD matter created. We emphasize the importance of the coarse-graining scale
required for the hydrodynamic modeling which determines the space-time
resolution and the associated limitations of collective flow observables. We
show that certain kinds of observables can indicate the degree of inhomogeneity
of the initial condition under less stringent condition than the local thermal
equilibrium subjected to the coarse-graining scale compatible to the scenario.
