Have you ever seen what happens when you walk through snow, mud, or wet sand?  How about if you go down the sidewalk with wet feet?  You leave your footprints behind.  If anyone looks at them, they are clues that you have been there.  After a while, the snow blows away, the sand and mud get washed away, and the wet footprints dry up.  The clues are lost.

Which way did they go?

Dr d12 at the English-language Wikipedia, CC BY-SA 3.0 , via Wikimedia Commons

Some clues do not go away so quickly.  They have been around for thousands, millions, or even billions of years!  These clues are called fossils: signs in stone of living things from the past.  They can take many forms, such as the shapes of dead plants or animals.  You could see the print of a leaf and its veins or an animal and its bones.  They might be parts from a living thing, such as eggs or roots.  Also, the signs could show how an animal lived, like a hole in the ground.  The organism, or living thing, is long gone, but there's still a sign in stone that it was once alive.

I'm feeling a little stuck in the mud today.

Ytrottier, CC BY-SA 3.0 , via Wikimedia Commons

How were fossils made?  Very slowly.  It happens in the same way if it's a print, a piece of a living thing, or part of its home.  For instance, suppose an organism died.  When most living things die, they just decay into tiny pieces that the wind and rain carry away.  Sometimes the body or print was protected.  Have you ever put a blanket on yourself to keep warm at night?  You were protecting yourself from the cold with a cover.  That's a lot like how the stone clues were made.  Sediment is another word for dirt, sand, and tiny rocks.  Most of the time, sediment was blown or washed over the body of the organism.  This layer protected the organism or what it left behind.     

Once an organism dies, many things can help it to decay.  Large animals and insects may eat parts of it.  You may have seen ants slowly eating a dead bug or animal.  What you can't see are the even smaller living things, like tiny cells of bacteria, that are doing the same thing.  They can get in very small spaces and break things down that are too far under the ground for the ants to reach.  Wind and rain can also help break down the body.  If the layer of sand and rock does not cover the organism quickly, these things will break the organism into tiny pieces until there is no sign that it was ever there!    

Now that was some wind storm.

BIT1982, CC BY-SA 3.0 , via Wikimedia Commons

The dirt cover alone is not enough.  Just like you, no matter how long you stay under your blanket, you are never going to turn to stone!  The sand and rock form a layer over the organism that slows down decay.  The soft parts, such as skin, will break down more quickly than harder parts such as bone.  Protected bone takes a very long time to go away.

While the bone is breaking down, water runs over and into it, carrying hard materials like iron with it.  This hard stuff fills in tiny holes in the bone.  Think of it like putting glue in a sponge.  There are lots of holes to fill in.  Once the glue dries, the whole sponge is much, much stronger than before.  If the soft sponge went away, you would still have the shape of it in hard glue.  You could still tell what the sponge looked like, even though all the real sponge is gone!

Finally, something I can use in the bath.

Johan, CC BY-SA 3.0 , via Wikimedia Commons

This all happens best when there is lots of water.  That is why the best signs of life from long ago are found where there used to be rivers, lakes, or oceans.  After a long time, these living things have fossilized.  If something is fossilized that means all of the organism is gone and only the hard, stone parts are left.  When we find these stone signs from a very long time ago, it means that over time, more and more dirt and rock covered the fossil and kept it safe.  Since the layers of the earth move slowly, after a long time, the fossil may come to the surface and be found.

Sometimes people have to dig into the earth to find these signs.  No matter how they are found, they can give us ideas of what life was like.  The fossil record is the name for all the stone signs in the world and all the information about life in the past.  This can tell us what animals and plants were alive long ago and where they lived.  We can figure out which living things were around at the same time and which are much older or younger.

Snail fossils always make a good first impression.

Jim Conrad, Public domain, via Wikimedia Commons

Your snowy footprints will not tell anyone much about what happened to you.  Maybe someone can tell which way you walked and if you were alone or not.  However, with these stone signs, we can learn a great deal about Earth's past.  We can discover what life came first and how it changed over the many years.  As we find more clues, we will understand more about the Earth and its life forms.  Who knows what new information is waiting for us under layers of rock and dirt?