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The air around us, and the water beneath the surface, is a mix of gases in constant motion. Pick an environment below and watch its composition come alive as a field of drifting particles, each colour a different gas, each in proportion to how much of that gas is actually present.
Surface air, the air we breathe, is overwhelmingly nitrogen and oxygen, with a thin scattering of argon and a trace of carbon dioxide and other gases. The particles above drift in the same proportions: for every 100 particles, about 78 are nitrogen and 21 are oxygen, so the field looks mostly like the two dominant colours with occasional flecks of the rest.
The major gases, nitrogen and oxygen, stay in almost the same ratio from sea level up through the stratosphere; the air simply gets thinner, not chemically different, until you reach very high altitudes. What changes noticeably is the trace component: carbon dioxide rises sharply in polluted cities, ozone concentrates in the stratosphere, and water vapour varies enormously with weather and location.
Underwater tells a completely different story. Water doesn't contain "air", it contains dissolved gases, and their balance is almost the reverse of the atmosphere in importance. Oxygen dissolved in seawater is what fish breathe, and it becomes scarce in the deep sea, where sunlight never reaches and photosynthesis can't replenish it. Dissolved carbon dioxide and nitrogen behave differently again, governed by pressure, temperature and biology rather than the simple mixing that rules the open air.
Composition drives climate, weather and life. The trace gases, the specks in the animation, not the dominant colours, are the ones that trap heat, form the ozone layer, and signal pollution. A change of a few particles per ten thousand in carbon dioxide is small on screen but enormous in effect. Watching the proportions shift between environments is a way of seeing, at a glance, why "the atmosphere" is really many different atmospheres stacked and layered across the planet.