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Atoms exist in different forms or compounds at different times in history and cycle between them. We can see this cycling in the element carbon and the compound water. Just as rocks can cycle between igneous, sedimentary and metamorphic, carbon and water can exist in different forms at different times.

Other elements and compounds also exist in cycles. Many humans eat protein in the form of meat from other animals. Our bodies break this down into its constituent parts called amino acids and then use these to make proteins within our own bodies for growth and repair. When we eventually die these building blocks are returned to the environment to be used by other living organisms.

Decomposing bacteria and fungi help dead organisms break down and rot. They help recycle minerals and nutrients to the environment, which can then be used by other organisms. As they decompose dead matter, the decomposers also respire and so release carbon dioxide to the environment, contributing to the carbon cycle.