Most of the energy we use in this country comes from sources far beneath the Earth's surface called fossil fuels. Petroleum (which is used to make oil and gasoline), natural gas and coal formed deep underground over hundreds of millions of years. These deposits were created as once-living plants and animals decomposed under high pressure and heat. Humans have created sophisticated ways to get these fuels out of the earth, and they now account for more than 80 percent of all the energy Americans use to heat our homes, cook food and drive our cars. But someday, these deposits of fossil fuels will run out. So experts want to come up with alternative sources of energy that are 'renewable,' which means they won't run out. Here are a couple of interesting renewable energy sources that have been growing quickly. Wind power: Do you remember seeing old grain windmills on a field trip or in storybooks? They used the wind to move giant blades, and as they turned, huge gears inside the windmill operated grain-crushing machinery. Modern windmills, called wind turbines, are used to generate electricity. Rather than turning grain machinery, the gears attached to a wind turbine's rotors move wires inside a magnetic field, which causes an electric charge to form in the wire. Constant movement of these wires creates an electric current that can be used to power homes and businesses. Companies are now building huge wind farms, but they can be placed only in areas that are consistently windy. Wind power now accounts for 1.5 percent of all electricity used around the world, and many more wind farms are planned. Garbage fuel: Most of the trash we throw out ends up in landfills _ huge, rotting piles of garbage. As this waste rots over time, it lets off a gas called methane, which is the main ingredient in natural gas. Companies are building plants that capture the methane from garbage dumps and use it to manufacture natural gas. When chilled to extremely low temperatures, natural gas turns into liquid, which takes up far less space and is easier to move around. In California, the trash collecting company Waste Management has built the largest plant in the world for turning methane from landfills into liquid natural gas. That fuel is then used to power hundreds of the company's garbage trucks. There's another benefit to this technology: Methane contributes to global warming, so this process keeps it from seeping into the atmosphere from the garbage dump. There are more than 500 landfills in the nation trying to recycle waste into energy. It's expensive to do, but experts say it will become cheaper as more companies do it and the technology improves. 