Running low on fuel? Just zip to the gas station and fill up your tank. The only trouble is, you won't be able to do that forever because Earth itself is running low on fuel. Most of the energy we use comes from fossil fuels like oil, gas, and coal, which are gradually running out. Not only that, using these fuels produces air pollution and carbon dioxide—the gas most responsible for global warming. If we want to carry on living our lives in much the same way, we need to switch to cleaner, greener fuel supplies—renewable energy, as it's known. This article is a brief, general introduction; we also have lots of detailed articles about the different kinds of renewable energy you can explore when you're ready.