When an animal or plant dies it is usually completely destroyed - either by another animal eating it or by decaying on the ground or in the water. A fossil can be made by an animal getting buried before it is destroyed. When that happens and the conditions are right, the animal remains are preserved in a fossil.
A simple definition of a fossil is: It is when the animal remains are naturally preserved, and it lived in the geological past.
These are really important facts about fossils. A fossil represents the remains or traces of a once living organisms. Most fossils are the remains of extinct organisms- that is, they belong to plants or animals that are no longer living anywhere on earth.