  When smooth granular material is fluidized by vertically shaking a container,
we find that the temperature in the direction of energy input always exceeds
the temperature in the other directions. An analytical model is presented which
shows how the anisotropy can be traced back to the inelasticity of the
interparticle collisions and the collisions with the wall. The model compares
very well with molecular dynamics simulations. It is concluded that any
non-isotropic driving of a granular gas in a steady state necessarily causes
anisotropy of the granular temperature.
