It may not sound pleasant, but waste products like used cooking oil and grease can be used as cheap feedstocks. They are attractive as fuel sources because they're both environmentally friendly and inexpensive. The issues regarding land use, energy consumption and environmental impact that often surround growing feedstocks like corn, sugarcane and palm trees, can be eliminated by using something that already exists and needs to be disposed of anyway. "Yellow grease," a term for recycled cooking oil that may include soy, palm or canola oil, is one type of waste product being used as fuel. Of course it's not possible to simply pour used restaurant grease into your car's gas tank; it must first be processed to remove impurities and turned into clean-burning fuel.