When an animal or plant dies, usually the remains are eaten or decompose. On occasion though, they get buried by mud or sediment, and in the right conditions, they harden into fossils.
The bones or original material decay, and water or other things seep into the impression left by the original material. This water and minerals continue to dissolve the original material, replacing it, and then they harden over time.
Once the fossil is formed, it’s usually heavy and feels more like a rock. It’s also colored like a rock because that’s what it really is! It just has a shape of something that was alive a long time ago, and that’s what makes it a fossil.
Picture of frog skeleton by Rhett
Craft Fun: Take some clay (plaster of paris works best). Mold it into a rounded flat circle. Press a leaf or some other object into it so that it leaves an imprint. Let your "fossil" harden, then show it to a friend and see if he or she can guess what it's a fossil of!
