The carbon cycle is a natural and integral part of life on Earth. A single carbon atom stored in a blade of grass may shift to the body of an animal that eats the grass. When the animal dies, its body might rot, and the carbon atom could join with oxygen to form CO2 in the air. From there it might be taken in by a tree in the process of photosynthesis and used as a building block in a branch or trunk, or absorbed by the ocean. And so on.