Fossils are the preserved remains, or traces of remains, of ancient organisms. Fossils are not the remains of the organism itself! They are rocks. 
 
A fossil can preserve an entire organism or just part of one. Bones, shells, feathers, and leaves can all become fossils. 
 
Fossils can be very large or very small. Microfossils are only visible with a microscope. Bacteria and pollen are microfossils. Macrofossils can be several meters long and weigh several tons. Macrofossils can be petrified trees or dinosaur bones.
 
Preserved remains become fossils if they reach an age of about 10,000 years. Fossils can come from the Archaeaean Eon (which began almost 4 billion years ago) all the way up to the Holocene Epoch (which continues today). The fossilized teeth of wooly mammoths are some of our most "recent" fossils. Some of the oldest fossils are those of ancient algae that lived in the ocean more than 3 billion years ago. 
 
Fossilization
 
The word fossil comes from the Latin word fossus, meaning "having been dug up." Fossils are often found in rock formations deep in the earth. 
 
Fossilization is the process of remains becoming fossils. Fossilization is rare. Most organisms decompose fairly quickly after they die. 
 
For an organism to be fossilized, the remains usually need to be covered by sediment soon after death. Sediment can include the sandy seafloor, lava, and even sticky tar.
 
Over time, minerals in the sediment seep into the remains. The remains become fossilized. Fossilization usually occur in organisms with hard, bony body parts, such as skeletons, teeth, or shells. Soft-bodied organisms, such as worms, are rarely fossilized. 
 
Sometimes, however, the sticky resin of a tree can become fossilized. This is called fossilized resin or amber. Amber can preserve the bodies of many delicate, soft-bodied organisms, such as ants, flies, and mosquitoes.
 
Body Fossils and Trace Fossils
 
The fossils of bones, teeth, and shells are called body fossils. Most dinosaur fossils are collections of body fossils. 
 
Trace fossils are rocks that have preserved evidence of biological activity. They are not fossilized remains, just the traces of organisms. The imprint of an ancient leaf or footprint is a trace fossil. Burrows can also create impressions in soft rocks or mud, leaving a trace fossil.
 
Paleontologists
 
Paleontologists are people who study fossils. Paleontologists find and study fossils all over the world, in almost every environment, from the hot desert to the humid jungle. Studying fossils helps them learn about when and how different species lived millions of years ago. Sometimes, fossils tell scientists how the Earth has changed.
 
Fossils of ancient marine animals called ammonites have been unearthed in the highest mountain range in the world, the Himalayas in Nepal. This tells scientists that millions of years ago, the rocks that became the Himalayas were at the bottom of the ocean.
 
Fossils of an ancient giant shark, a megalodon, have been found in the landlocked U.S. state of Utah. This tells scientists that millions of years ago, the middle of North America was probably entirely underwater.
fossil
Juvenile pterodactyls like this one are called "flaplings."

Mary Anning
The 19th-century British fossil collector Mary Anning proved you don't have to be a paleontologist to contribute to science. Anning was one of the first people to collect, display, and correctly identify the fossils of ichthyosaurs, plesiosaurs, and pterosaurs. Her contributions to the understanding of Jurassic life were so impressive that in 2010, Anning was named among the ten British women who have most influenced the history of science.

Microfossils
Even though most of us have only seen dinosaur fossils in museums, most fossils are not that big. Some of them are so small, you can't see them without a microscope.

algae
Plural Noun

(singular: alga) diverse group of aquatic organisms, the largest of which are seaweeds.

amber
Noun

translucent, yellow-orange material made of the resin of ancient trees. Amber is sometimes considered a gemstone.

ancient
Adjective

very old.

Plural Noun

(singular: bacterium) single-celled organisms found in every ecosystem on Earth.

body fossil
Noun

preserved evidence of what was once the body of an ancient organism, such as bones or teeth.

decompose
Verb

to decay or break down.

Noun

area of land that receives no more than 25 centimeters (10 inches) of precipitation a year.

Noun

remnant, impression, or trace of an ancient organism.

fossilize
Verb

to become a solid mineral.

humid
Adjective

air containing a large amount of water vapor.

jungle
Noun

tropical ecosystem filled with trees and underbrush.

Latin
Noun

language of ancient Rome and the Roman Empire.

lava
Noun

molten rock, or magma, that erupts from volcanoes or fissures in the Earth's surface.

macrofossil
Noun

fossil that is large enough to be seen and analyzed without a microscope.

mammoth
Noun

one of many extinct species of large animals related to elephants, with long, curved tusks. The last mammoths became extinct about 5,000 years ago.

megalodon
Noun

extinct shark that lived between 25 million and 1.5 million years ago.

microfossil
Noun

fossil that can only be seen and analyzed with a microscope, such as a grain of pollen or a single bacterium.

mineral
Noun

inorganic material that has a characteristic chemical composition and specific crystal structure.

organism
Noun

living or once-living thing.

paleontologist
Noun

person who studies fossils and life from early geologic periods.

pollen
Noun

powdery material produced by plants, each grain of which contains a male gamete capable of fertilizing a female ovule.

remains
Noun

materials left from a dead or absent organism.

resin
Noun

clear, sticky substance produced by some plants.

Noun

solid material transported and deposited by water, ice, and wind.

seep
Verb

to slowly flow through a border.

shell
Noun

hard outer covering of an animal.

skeleton
Noun

bones of a body.

tar
Noun

dark, sticky petroleum product created from the decomposition of organic material such as wood.

trace fossil
Noun

preserved evidence of the presence or behavior of an ancient organism, such as tracks, feces, or burrows.