  In relativistic heavy-ion collisions, the system has gone through a series of
evolution, almost at every stage of its evolution it leaves behind footprints
in flow observable. Those footprints contain valuable information of the bulk
property of the (nearly) perfect liquid. By examing footprints of the nearly
perfect liquid, we address a few important issues, including the ideal
hydrodynamic limit, estimation of $\eta/s$, testing the Number of Constituent
Quark scaling at low energy, in small system, at large transverse momentum, and
in forward region. Future prospect of flow study is discussed.
