Oil, gas, and coal are called "fossil fuels" because they come from fossils of ancient plants and animals. Those plants and animals had carbon in them. When we burn oil, gas, and coal, the carbon becomes carbon dioxide and goes into the air. This makes the greenhouse effect, which means that the carbon dioxide makes the earth warmer, like a greenhouse. A carbon footprint is the carbon dioxide that is put into the air because of a person, a group of people, an event or a product.