  We develop a new theory of perfect fluids with translation and rotation
symmetry, which is also applicable in the absence of any type of boost
symmetry. It involves introducing a new fluid variable, the kinetic mass
density, which is needed to define the most general energy-momentum tensor for
perfect fluids. Our theory leads to corrections to the Euler equations for
perfect fluids that might be observable in hydrodynamic fluid experiments. We
also derive new expressions for the speed of sound in perfect fluids. Our
theory reduces to the known perfect fluid models when boost symmetry is
present. It can also be adapted to (non-relativistic) scale invariant fluids
with critical exponent $z$. We show that perfect fluids cannot have
Schr\"odinger symmetry unless $z=2$. For generic values of $z$ there can be
fluids with Lifshitz symmetry, and as a concrete example, we work out in detail
the thermodynamics and fluid description of an ideal gas of Lifshitz particles
and compute the speed of sound for the classical and quantum Lifshitz gasses.
