The pooling equilibrium

During the Nazi occupation of Paris:

Germans spent a good deal of their free time in the bathhouses and swimming pools of Paris for the same reason: “In a swimsuit, no one could tell the difference between a German and a Frenchman.”

That is from the new and excellent When Paris Went Dark: The City of Light Under German Occupation, 1940-1944, by Ronald C. Rosbottom.