Azimuthal anisotropy of final particle distributions was originally
introduced as a signature of transverse collective flow. We show that finite
anisotropy in momentum space can result solely from the shape of the particle
emitting source. However, by comparing the differential anisotropy to recent
data from STAR collaboration we can exclude such a scenario, but instead show
that the data favour strong flow as resulting from a hydrodynamical evolution.Comment: To appear in proceedings of Quark Matter 2001, 4 pages LaTeX, uses
espcrc1.st