Physicists who work on superfluidity have often “lost their cool” arguing over the meaning or even the existence of certain new effects. Controversy arose because measurements were frequently rough and inconclusive. Now a simple but powerful new technique for studying processes that affect superfluid film flow has been developed, which uses level sensing to probe the chemical potential along flowing liquid He II film. The work, reported in the 7 November issue of Phys. Rev. Letters, was done by William E. Keller and Edward F. Hammel of Los Alamos Scientific Laboratory, who acted on a suggestion made by Brian D. Josephson of the University of Illinois.