Biofuels are liquid fuels produced from renewable biological sources, including plants and algae. They are substitutes for conventional fossil fuels, such as petroleum, propane, coal, and natural gas. Biofuels can be produced from any carbon source that can be replenished rapidly, such as plants. The two most common types of biofuels in use today are ethanol and biodiesel, both of which represent the first generation of biofuel technology. Ethanol is an alcohol fuel that is blended with petroleum gasoline for use in vehicles, while biodiesel is a liquid fuel produced from renewable sources, such as new and used vegetable oils and animal fats, and is a cleaner-burning replacement for petroleum-based diesel fuel. Other biofuels include renewable heating oil, renewable jet fuel, methane gas, and biogas. To avoid a "food versus fuel" dilemma, second-generation biofuels (also called advanced biofuels or sustainable biofuels) are made from waste products, such as rice straw, rice husk, wood chips, and sawdust. Biofuels can reduce greenhouse gas emissions and increase energy security by providing an alternative to fossil fuels. By 2050, biofuels could reduce our greenhouse gas emissions by 1.7 billion tons per year—equivalent to more than 80% of current transportation-related emissions.