  Starting from a standard description of an ideal, isentropic fluid, we derive
the effective theory governing a gapless non-relativistic mode---the sound
mode. The theory, which is dictated by the requirement of Galilei invariance,
entails the entire set of hydrodynamic equations. The gaplessness of the sound
mode is explained by identifying it as the Goldstone mode associated with the
spontaneous breakdown of Galilei invariance. Differences with a superfluid are
pointed out.
