Two bottles of different-colored water are placed vertically, one on top of the other with open ends together. They remain stable if the bottle with the hotter (red-colored) water is placed on top of the bottle with the cooler (blue-colored) water. If they are arranged in the reverse order, mixing occurs rapidly to form the color purple, which becomes uniformly distributed within the two bottles. Thermal-induced density differences cause “heavier” cooler fluids to fall and push warmer “lighter” fluids up, creating circulation in convection cells. This activity could be used either for diagnostic (Engage-phase) or formative (Elaborate-phase) assessment of students’ understanding of the topics of fluid density and/or thermal-driven convection cells.