FOSSIL: A rock that was created by minerals that saved the picture of animals and plants of long time ago.
The minerals slowly take the place of the organism or plant and become hard all together like a statue of what it was before.
Where there was a fish that was buried in the mud, the fish disappears, and the mud and minerals go into the space where the fish was before.
When you find a fossil, it will be inside other rocks and minerals. But because the fossil is almost 1 piece, it usually falls away from the other rock around it.
Then, you are left with just the fossil, looking like a statue of the old, gone animal or plant.