  Stretching and folding dynamics in the incompressible, stratified 3D Euler
and Navier-Stokes equations are reviewed in the context of the vector $\bdB =
\nabla q\times\nabla\theta$ where $q=\bom\cdot\nabla\theta$. The variable
$\theta$ is the temperature and $\bdB$ satisfies $\partial_{t}\bdB =
\mbox{curl}\,(\bu\times\bdB)$. These ideas are then discussed in the context of
the full compressible Navier-Stokes equations where $q$ takes the two forms $q
= \bom\cdot\nabla\rho$ and $q = \bom\cdot\nabla(\ln\rho)$.
