Rock Strata is layers of sedimentary rock that are clearly different from each other. You could see how different they are by the different colors of each layer.
How do we know how old fossils are?
Scientists figure out how old a fossil is by the clues fossils give when they are in sedimentary rock. They could also find out how old fossils are because the fossil had to be fossilized in the same time span when the rock was formed.