Making a fossil is both easy and hard. Fossils are essentially the remains of life in the earth. A human bone is technically a fossil, though no one would necessarily look at it that way. Creating fossils takes hundreds, thousands, and millions of years. Typically, fossils were created when organic materials came to rest in muddy areas which dried up. Imagine that a dinosaur fell into a mud puddle, broke its leg, and died. Within a few weeks, this mud puddle hardened to form a sort of rock. Inside, the dinosaur rotted, leaving only its bones. These bones formed a shape in stark contrast to the surrounding rock. Over time, the bones rotted to leave a hollow shell in the muddy rock. Eventually, over millions of years, new chemicals crept through the muddy rock (typically through rainwater) to fill this void. This substance hardened. Digging through the old muddy rock today, a paleontologist discovers the fossil–hardened rock in the exact shape of old dinosaur bones.