A fossil is the preserved remains or traces of ancient living things, usually found in rock. When a dinosaur died millions of years ago and its bones turned to stone over time, those bones became fossils. When an ancient fern left an imprint in mud that hardened into rock, that impression became a fossil too.

Fossils form through a remarkable process. After a plant or animal dies, it must be buried quickly, often in mud or sand. Over millions of years, minerals seep into the remains, slowly replacing the original material and turning it into rock. This is why we find stone versions of bones, shells, and even dinosaur footprints.

Scientists called paleontologists study fossils to learn about life in Earth's distant past. They've discovered that whales once had legs, that massive ferns covered the planet before flowering plants existed, and that the Sahara Desert was once an ocean teeming with life. Each fossil is like a window into a world that vanished long before humans existed.

The word can also describe someone or something hopelessly outdated. If your friend calls your flip phone a fossil, they mean it's ancient by today's standards. Fossil fuels like oil and coal get their name because they formed from plants and animals that died millions of years ago.