Outline the method for dating rocks and fossils using radioisotopes
So if you take a sample of, say, a carbonate rock like limestone (which is made from the deposited shells of myriad tiny dead sea creatures), you can look at the carbon atoms and the oxygen atoms in it, see what proportions of them are still radioactive isotopes, and do some fairly easy math to see how many half-lives of those elements have passed since the critter whose body MADE the carbonate, died.
Obviously, things with short half-lives aren't useful for dating things that happened a really long time ago (enough half-lives will have passed so that they're all nonradioactive and will stay that way).
Distinctive fossils are one way we can match up fossil layer-cake diagrams from one area to another, because if a fossil only ever lived at one time, then you can line up the diagrams and see which other layers are consistently older, or consistently younger, than that distinctive layer.
So far, so good, but that only gives RELATIVE dating (which thing is always older than what other thing -- like an alphabetic order, although not all areas have all layers of rock in them). Because we have such good, large datasets about differing relative dating, you can get pretty close on other layers if you can absolute-date just one or two layers in the cake. Let's say you have a single atom of a radioactive isotope, say carbon-14.
So if you draw a diagram of what the layer-cake looks like near where you live, and then go fifty miles away and draw what THAT layer-cake looks like, and so on, you've collected a bunch of specific samples of the history of the rocks in your area, like looking at tree rings.
When they die, the timer starts ticking, and all the radioactive isotopes remaining in the corpse start to decay at their predictable rates.
The earliest birds appear after dinosaurs but before modern birds. In fact, “out-of-sequence” fossils are found routinely.