Fossils are the remains of prehistoric plants and animals. However, not all plants and animals become fossils. Most creatures on Earth do not become fossils. This is because a very specific sequence of events must happen in order for a fossil to form.
First, the creature will usually die in or near water. Second, while the soft parts of an animal can be eaten by scavengers and bacteria, the hard parts of the body cannot be disturbed. Since most animals were not careful while eating, many creatures who were eaten did not become fossils.
Next, over time sediments will cover the creature. Sediments are small pieces of rock and other materials. An example of a sediment is sand. These sediments pile up until the original creature is far underground. This can take millions of years.
Eventually, water will seep through the sediments carrying dissolved minerals. As the water dissolves the bones and other hard parts of the creature, the minerals will replace them. This makes a copy of the bones and other hard parts that is as strong as rock.
Over time, the pressure from the layers of sediments above the creature will turn the area around it into sedimentary rock. The sedimentary rock will stay in place for millions of years. Slowly the rock above the creature is weathered and eroded away.
Weathering is the process of breaking down rock into smaller pieces. Water and wind are two things that weather rocks. Erosion is the process of moving sediments from one place to another. Once enough rock is weathered and eroded, a part of the fossil may become visible.
A paleontologist, a scientist who studies fossils, will carefully dig out the rest of the fossil. He or she will try to put the bones of the fossil back together to recreate the skeleton of the creature who died. The bones of the fossil can tell scientists a lot about the creature who died and the world it lived in long ago.