A: The carbon cycle is the circulation and transformation of carbon back and forth between living things and the environment. Carbon is an element, something that cannot be broken down into a simpler substance. Other examples of elements are oxygen, nitrogen, calcium, iron, and hydrogen. Carbon compounds are present in living things like plants and animals and in nonliving things like rocks and soil. Carbon compounds can exist as solids (such as diamonds or coal), liquids (such as crude oil), or gases (such as carbon dioxide). Carbon is often referred to as the “building block of life” because living things are based on carbon and carbon compounds.