  A recent granular rheology based on an implicit `granular fluidity' field has
been shown to quantitatively predict many nonlocal phenomena. However, the
physical nature of the field has not been identified. Here, the granular
fluidity is found to be a kinematic variable given by the velocity fluctuation
and packing fraction. This is verified with many discrete element simulations,
which show the operational fluidity definition, solutions of the fluidity
model, and the proposed microscopic formula all agree. Kinetic theoretical and
Eyring-like explanations shed insight into the obtained form.
