Fossil fuels are decomposed bits of matter that have been trapped underneath the earths surface for thousands of years. These include coal, natural gas, and oil. All three of them are used as fuel, starting during the industrial revolution. Usually, they start as dead animals and plants. Millions of years of heat and pressure turn them into fossil fuels. We use them as fuel because they release heat energy when burned; they also release Carbon Dioxide. To learn more about the dangerous effects of Carbon Dioxide, click here. Fossil fuels are nonrenewable resources, meaning that they cannot be reproduced at a reasonable rate.