Oil is a thick, slippery liquid that doesn't mix with water. When you pour oil and water together, they separate into layers, with oil floating on top.

The most common type of oil comes from deep underground, formed over millions of years from ancient plants and sea creatures. This crude oil (also called petroleum) gets refined into gasoline for cars, diesel for trucks, and heating fuel for homes. For over a century, oil has powered most of the world's transportation, making it one of the most valuable resources on Earth. Countries with large oil reserves often become wealthy from selling it.

Plants also produce oils. Olive oil, pressed from olives, has been used in cooking for thousands of years. Vegetable oil comes from plants like corn, sunflowers, or soybeans. These oils add flavor and help food cook evenly.

Oil also keeps machines running smoothly. When metal parts rub together, they create friction and heat. A few drops of motor oil or machine oil coat the parts so they slide past each other easily, preventing wear and overheating. Without oil, car engines could seize up in minutes.

The word can be a verb too: when you oil a squeaky door hinge, you're applying oil to make it move quietly again.