How does a fossil help identify things that lived in the past?

A fossil is the mineral formation left as slowly decaying matter, such as bones or shells, is replaced with mineral deposits, or impressions left behind when a soft substance such as mud dries into a solid rock-like state while still holding the imprint.

These help identify things that lived in the past by giving us a record of what the skeletal structure of animals looked like, or other features, such as the shells or footprints. From this data, we can extrapolate a lot about the creatures. The teeth show if they were carnivores or herbivores. The skeleton shows the general shape of the creatures. The size of the brain can be determined by looking at the cranial cavity (the space within the skull that holds the brain)

There are many other things that can be determined by studying fossils, but there is not enough room to include everything here.