The Earth is a living active planet, always changing, and functions through various energy and chemical cycles. There are continual interactions between the biosphere (life), lithosphere (land), hydrosphere (water), and atmosphere (air) during these cycles. Various substances on Earth move endlessly throughout these four spheres. Of the four spheres, the atmosphere transports elements the fastest. Water, for example, evaporates into the atmosphere from both the land and the ocean, where it condenses and falls back to the land and oceans, where it is then used by plants, animals, and people, fills rivers, reservoirs, and cycles through lakes. During the lifetime of a water molecule, it changes states (liquid, solid, or vapor) many times and can alternately exist in the form of rain, snow, ice, steam, sleet, hail, and water vapor. Over time, that same water molecule may have occurred in a blizzard, a glacier, a tornado, a hurricane, a tsunami, a flood, a simple spring shower, or a glass of water at a dinner table.
The substances may exist in different forms depending on which sphere they are in, such as water can be a liquid in the ocean, a solid on land, and a vapor in the atmosphere. The substances can also be used by specific organisms at different times in the cycle. The important concept is that the substances are constantly moving through the systems—sometimes quickly (within hours, such as a rainstorm) or slowly (taking thousands of years, such as in a glacier).
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