Here's why:

* Sedimentary rocks form from the accumulation and cementation of sediments (like sand, mud, or organic matter). These sediments are deposited in layers, with the oldest layers typically found at the bottom.

* Igneous rocks form from cooled magma or lava, and their position in rock layers isn't necessarily related to age.

* Metamorphic rocks form when existing rocks are transformed by heat and pressure, and their original position in rock layers might be obscured.

Let me know if you have any other clues about yourself!