The law that states that the youngest layer of a rock is on top and the oldest on bottom, and with layer being younger than the one beneath it and older than the one above it.
An example of a fossil is a once living thing that has been incased in some sort of fluid that has been hardened, so it could have been a sort of sap, but you are able to see it still, after thousands or even million of years, because of the fact that it has been preserved.
WHAT FOSSILS CAN TELL US ABOUT HOW LIFE HAS CHANGED
By studying fossils, we can tell how long life has existed on Earth, and how different plants and animals are now, compared to when dinosaurs and other extinct creatures roamed our earth.