There have been many oil spills over the years. Perhaps you heard or learned about the Gulf oil spill in the U.S. that happened in April 2010? A spill like this that is close to land causes many problems for the environment and makes it difficult to clean up. As little as three gallons of oil can spread to make a slick mess covering one acre of the ocean's surface. With the Gulf oil spill, it's estimated that 200,000 gallons a day spilled into the ocean. Oil spills like this are very damaging, but they aren't the only source of oil that is polluting our waters. Rain washes particles from air pollution into the ocean. And one of the biggest sources of oil polluting is from the oil people put down their drains every day or runoff from parking lots. Oil and water don't mix—perhaps you have heard this before? And you probably know that oil is sticky and greasy. This makes it even more difficult to clean up. Let's take a look at the chemical properties of oil and water to see why. Each water molecule is made of two hydrogen atoms and one oxygen atom - H2O. When the two hydrogen atoms bond with the oxygen, they attach to the top of the molecule, rather like Mickey Mouse ears. This molecular structure gives the water molecule polarity, or a lopsided electrical charge that attracts other atoms. Because of their polarity, water molecules are strongly attracted to one another. This also gives water its unique properties. Oil is made of more complex molecules, containing carbon and hydrogen. Oil molecules are non-polar, meaning they don't stick together like water molecules do. Oil is thick and heavy, yet its molecules are spread farther apart, lowering the density. Because it has a lower density, oil floats on water's surface.