  I consider a simple set of equations that govern the expansion of
boost-invariant plasmas of massless particles. These equations describe the
transition from a collisionless regime at early time to hydrodynamics at late
time. Their mathematical structure encompasses all versions of second order
hydrodynamics. We emphasize that the apparent success of Israel-Stewart
hydrodynamics at early time has little to do with ``hydrodynamics'' proper, but
rather with a particular feature of Israel-Stewart equations that allows them
to effectively mimic the collisionless regime.
