Fossils are made when a living thing dies and is buried by mud, silt, volcanic ash, or sand.  Fossils could also be frozen in ice, mummified in hot or cold deserts, or preserved in tar.  Usually, all of a living thing's soft parts decay, leaving only the hard parts to be buried, except when a living thing is frozen or mummified. The mud, silt, and sand are called sediments.
    The sediments have water with minerals in it.  The minerals in the water soak into the hard parts, changing them into a rock like material and preserving the hard parts as fossils.
    One kind of fossilization is called replacement, where the minerals have replaced, molecule by molecule, the hard parts of the remains.
   Permineralization is another kind of fossilization. Permineralization happens when  the minerals have only filled in the spaces of the hard parts of the remains.
    Living things which die in or near oceans, lakes, or rivers have a better chance of fossilization than those which die on dry land, because they will be quickly buried.  Over thousands and millions of years the sediments form heavy layers which slowly turn into sedimentary rock.

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