  We examine the question of whether fluids and crystals are differentiated on
the basis of their zero frequency shear moduli or their limiting zero frequency
shear viscosity. We show that while fluids, in contrast with crystals, do have
a zero value for their shear modulus, in contradiction to a widespread
presumption, a crystal does not have an infinite or exceedingly large value for
its limiting zero frequency shear viscosity. In fact, while the limiting shear
viscosity of a crystal is much larger than that of the liquid from which it is
formed, its viscosity is much less than that of the corresponding glass that
may form assuming the liquid is a good enough glass former.
