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Fossils are the preserved remains of plants or animals. For such remains to be considered fossils, scientists have decided they have to be over 10,000 years old. There are two main types of fossils, body fossils and trace fossils. Body fossils are the preserved remains of a plant or animal's body. Trace fossils are the remains of the activity of an animal, such as preserved trackways, footprints, fossilized egg shells, and nests.
When asked what a fossil is, most people think of petrified bones or petrified wood. Permineralization is a process. For bone to be permineralized, the body must first be quickly buried. Second, ground water fills up all the empty spaces in body, even the cells get filled with water. Third, the water slowly dissolves the organic material and leaves minerals behind. By the time permineralization is done, what was once bone is now a rock in the shape of a bone. Unlike what you see in cartoons, dogs wouldn't be interested in these bones.
When an animal or plant dies, it may fall into mud or soft sand and make an impression or mark in the dirt. The body is then covered by another layer of mud or sand. Over time, the body falls apart and is dissolved. The mud or sand can harden into rock preserving the impression of the body, leaving an animal or plant shaped hole in the rock. This hole is called a mold fossil. If the mold becomes filled over time with other minerals the rock is called a cast fossil.
A simple experiment can show you how this works. Take some clay and press a seashell or some other object into the clay. Pull the sea shell out of the clay any you will see a detailed impression of your seashell in the clay. If, over time, the clay hardens into rock the result would be a fossil mold. But really, who has millions of years to wait to make their own fossil? Here's the quick way. Pour plaster of Paris, dental stone, or other plaster into the mold. Wait for it to harden and you have just made your own cast fossil.
Another type of fossil is a resin fossil. Resin is sometimes called amber. Plants, mostly trees, secrete sticky stuff called resin. Sometimes insects, other small animals, or bits of plants get stuck in the sticky resin. The resin hardens overtime and is preserved in rock making a fossil.