Carbon Cycle, the series of natural processes by which carbon in the air is made available to living things, is used by them, and is then returned to the air. Such food-making organisms as plants and algae need carbon to form carbohydrates, which are essential for growth. They absorb carbon dioxide from the air; through photosynthesis, the carbon dioxide is combined with water to form carbohydrates. Other living things, such as animals, need carbohydrates for energy, but, unlike plants, cannot manufacture their own. Herbivores obtain carbohydrates by eating green plants and metabolize (chemically break down) the carbohydrates into useful substances. Carnivores, in turn, obtain these useful substances by eating herbivores. Carbon dioxide is released back into the atmosphere when the animals breathe. Small amounts of carbon dioxide are released into the air by the decomposition of dead organisms by the action of certain bacteria and fungi. The burning of fossil fuels, such as coal, also releases carbon dioxide into the atmosphere.