  We discuss heat conductivity from the point of view of a variational
multi-fluid model, treating entropy as a dynamical entity. We demonstrate that
a two-fluid model with a massive fluid component and a massless entropy can
reproduce a number of key results from extended irreversible thermodynamics. In
particular, we show that the entropy entrainment is intimately linked to the
thermal relaxation time that is required to make heat propagation in solids
causal. We also discuss non-local terms that arise naturally in a dissipative
multi-fluid model, and relate these terms to those of phonon hydrodynamics.
Finally, we formulate a complete heat conducting two-component model and
discuss briefly the new dissipative terms that arise.
