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I learned that our atmosphere has ~78% Nitrogen and ~21% Oxygen. If I fill a glass with 21% water and 78% Canola Oil, the more dense (and thus heavier) water will take a full bottom later and Canola Oil will sit on top and take the rest of the space.
Why don't we see the same in our atmosphere? Troposphere is the bottom layer, Oxygen is Heavier than Nitrogen; but why is the composition of the bottom layer of our atmosphere still same as the overall composition? What obvious clue am I missing?