  We present an analytic solution of a simple set of equations that govern the
expansion of boost-invariant plasmas of massless particles. These equations
describe, approximately, the early time, collisionless regime, and the
transition to hydrodynamics at late time. Their mathematical structure
encompasses all versions of second order hydrodynamics when applied to Bjorken
flows. The analytic solution provides an explicit expression for the attractor
solution that connects the collisionless regime to hydrodynamics. It also
constitutes a neat example of an application of the theory of resurgence in
asymptotic series.
