Carbon dioxide - "a colorless, odorless gas. It makes up less than one percent of the Earth's atmosphere and is found in the atmospheres of other planets, notably Mars and Venus. Carbon dioxide is formed when compounds call carbonates (containing carbon and oxygen) are heated or treated with an acid and also when coal, oil, wood, and other carbon compounds are burned. Animals produce carbon dioxide and release it into the air when breathing out. Plants convert carbon dioxide into oxygen by photosynthesis. Carbon dioxide absorbs heat and traps it near the Earth's surface." From The Kingfisher Science Encyclopedia, edited by Catherine Headlam. New York: Kingfisher Books, 1993.