a scientist finds a rock with fossils at two different levels tell why he or she might conclude that the fossils on the bottom are older the the fossils on the top

I don't remember much, but I'll try my best.

Explanation:

Over time, sediments rocks pile up to form horizontal layers of sedimentary rocks. The bottom layer of rock forms first, which means it's the oldest. This would mean that while you can't be sure when each layer of sediments formed, each layer is younger than the one below it.