Stratified Flow A tank of water is salty and dense at the bottom, fresh and light on top. Jiggle a rod in the middle and the disturbance can only leave along four straight beams — a glowing X. Change how fast you jiggle and the beams tilt; jiggle faster than the fluid can bounce back and they vanish. The journey: gravity on the density steps gives a springiness, the buoyancy frequency N. Internal waves obey ω = N cos β, so a single drive frequency fixes one propagation angle — hence beams, not circles. Their strangest trick: the crests slide across each beam while the energy runs along it.