With two matching jars, red and blue food coloring, some hot and cold water, an index card, and a dish or baking pan, you can observe how the different densities of hot and cold water behave. This experiment by HooplaKidzLab demonstrates how the more dense cold water sinks — those molecules are closer together — pushing the slightly less dense hot water to float on top.
Air behaves in the same way… think of hot air balloons as an example, or a multi-story house on a hot day, where the top floors are warmer than the bottom floors or basement.
Watch the experiment again in this easy DIY six second science demo:
Related reading: Heat transfer and efficiency.
Related video experiment: How to make a hot air balloon.Thanks, @alba.
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