How does something become fossilized?
Fossilization occurs in a variety of ways, but usually fossils are formed when an animal or plant dies and is buried in sediment.
So, what are fossils?
Fossils are any trace of past life, they can include plant impressions, teeth, bones, fossilized skin, trackways, or any trace of a prehistoric organism.
Fossilization occurs when certain external conditions are met to produce the best resulting fossils. It should be noted that soft tissue like skin and cartilage do not typically fossilize very well. In exceptional cases fossilized skin impressions happen, but it's pretty rare.
For organic material to become fossilized it should be buried soon after death. This means marine animals and animals that lived near the shores or rivers preserve much better than animals that lived in dry desert environments. Which explains why some of the most common fossils are shelled invertebrates like ammonites, corals, and clams.
Millions of years ago when animals such as these sank to the bottom of a prehistoric ocean or a lake, they were quickly buried under sediment. Over millions of years the original minerals that made up the shells would be replaced by minerals in the surrounding rock, turning the fossil itself into a cast of the original organic material.
These changes explain why fossil bones are heavier and have different colors than modern bones!