  We analyse the recently measured $v_2$ fluctuation in the context of
establishing the degree of fluidity of the matter produced in heavy ion
collisions. We argue that flow observables within systems with a non-negligible
mean free path should acquire a "dynamical" fluctuation, due to the random
nature of each collision between the system's degrees of freedom. Because of
this, $v_2$ fluctuations can be used to estimate the Knudsen number of the
system produced at RHIC. To illustrate this quantitatively, we apply the UrQMD
model, with scaled cross sections, to show that collisions at RHIC have a
Knudsen number at least one order of magnitude above the expected value for an
interacting hadron gas. Furthermore, we argue that the Knudsen number is also
bound from above by the $v_2$ fluctuation data, because too large a Knudsen
number would break the observed scaling of $v_2$ fluctuations due to the onset
of turbulent flow. We propose, therefore that $v_2$ fluctuation measurements,
together with an understanding of the turbulent regime for relativistic
hydrodynamics, will provide an upper as well as a lower limit for the Knudsen
number.
