We get energy from sunlight, wind, nuclear plants, and damns. However, most of the energy that we use comes from fossil fuels. There are three major fossil fuels: Coal, oil and natural gas. These are composed of carbon, hydrogen, nitrogen, oxygen and sulfur. Carbon is the major element in fossil fuels. We obtain energy from fossil fuels by burning them. Coal is the most abundant fossil fuel in the U.S. Yet it has a major disadvantage. It is a strong pollutant. When it is burned, it releases impurities like nitrogen and sulfur, which combine with water vapor to create "acid rain". Also, coal is a major contributor to the imbalance of the carbon cycle. When coal burns, it forms carbon dioxide. Scientists believe that carbon dioxide traps the earth"s heat in a process called the "greenhouse effect". However, scientists have developed new methods to burn coal more efficiently, filter impurities before they are released in the atmosphere, and cut down on the release of carbon dioxide. Oil is another important fossil fuel. It is found under tons of rock, deep in the earth"s crust. Precisely, oil is found in rocks, trapped in little pores. When an oil well is dug in the earth, pressure from the reservoirs will make the oil spurt to the top of the well. When oil is pumped out of reservoirs, about only ¼ of the oil available will be accessible to the pumps. Scientists and engineers have yet to discover a method to collect the remaining ¾ of oil. Often, natural gas is found with oil. In some cases, oil companies separate the two fossil fuels and inject the natural gas back into the reservoir, creating enough pressure to allow more oil to spring to the surface. Natural gas is a colorless, odorless gas. It is responsible for fueling over half of the United States" homes" appliances. Some countries have discarded natural gas because they think it is worthless. Precisely, there isn"t enough natural gas to last very long in the future.