This activity demonstrates that fluids arrange into layers according to their densities. The two "water masses" - salt (blue or green) vs. fresh (yellow), or cold (blue or green) vs. warm (yellow) - are initially separated by the tank's divider. When the divider is removed, the denser water (salt water or cold water) sinks to the bottom of the container and the less-dense water (fresh or warm water) floats above, forming a stratified column. In the process, an internal wave is formed in the tank.

Stratification forms an effective barrier for the exchange of nutrients and dissolved gases between the top, illuminated surface layer where phytoplankton can thrive, and the deep, nutrient-rich waters. Stratification therefore has important implications for biological and biogeochemical processes in the ocean.

When one column of the apparatus shown at right is warmed and the other is cooled, density differences are created between the bottom of the vertical tubes, causing a pressure gradient to develop. Density differences cause water masses to sink or rise until they reach their density equilibrium level; once a water mass reaches its equilibrium density level, it begins to move horizontally in response to a pressure gradient. (Note: Pressure gradients result from differences in the vertical distributions of density, and hence hydrostatic pressure, between regions where water is denser or lighter.) Because the cold water is denser, it will move along the lower connecting tube; the hot water will move along the upper connecting tube. This activity provides a good illustration of density driven ocean circulation - for example, the global conveyor belt.