  Continuum mechanics (e.g., hydrodynamics, elasticity theory) is based on the
assumption that a small set of fields provides a closed description on large
space and time scales. Conditions governing the choice for these fields are
discussed in the context of granular fluids and multi-component fluids. In the
first case, the relevance of temperature or energy as a hydrodynamic field is
justified. For mixtures, the use of a total temperature and single flow
velocity is compared with the use of multiple species temperatures and
velocities.
