Thermally stratified water has a refractive index that varies with height. If an object passes through the water, the waters part at its front and rejoin at the rear. The rejoining waters are seldom in perfect register vertically, so the surface of confluence between them is marked by a jump in temperature and therefore in refractive index, and is visible in shadowgraphic projection. Two shadowgraphs in stereo display the surface in three dimensions. If the shape and motion of any surfaces of confluence are known, the motion of the fluid everywhere else can be computed. The observer seems automatically to do this calculation approximately in his head. As a cheap, convenient, mathematically elegant, and psychologically natural way of displaying flow phenomena, the method should have a place in the teaching of hydrodynamics.