  Elliptic flow in ultrarelativistic heavy-ion collisions results from the
hydrodynamic response to the spatial anisotropy of the initial density profile.
A long-standing problem in the interpretation of flow data is that
uncertainties in the initial anisotropy are mingled with uncertainties in the
response. We argue that the non-Gaussianity of flow fluctuations in small
systems can be used to disentangle the initial state from the response. We
apply this method to recent measurements of anisotropic flow in Pb+Pb and p+Pb
collisions at the LHC. The response coefficient is found to decrease mildly as
the system becomes smaller. This mild decrease is consistent with a low value
of the ratio of viscosity over entropy.
