Waste oil from truck and car engines causes huge environmental problems if you tip it down the drain. It pollutes our rivers and seas, the wildlife that depend on them, and even the water we drink. If you take your waste oil along to a recycling center, it not only keeps our waterways clean—it can also be reprocessed into new products such as heating oil. Waste vegetable oils (made by frying food, for example) can be turned into a useful kind of vehicle fuel called biodiesel.