One day the world will run out of fossil fuels, and with it, our main sources of energy will go up in smoke. But biofuels are different. They're made from plants that can grow and be replanted again and again. Perennial crops don't even have to be replanted. Still, not all biofuels are created equal. Some "energy crops" produce more energy than others. For example, rapeseed has a higher oil content than other vegetable plants, which means rapeseed can generate more energy when burned. Perennial plants, such as switchgrass, provide an abundant source of power, generating five times as much energy as they take to grow [source: Biello].