Here's why:

* Sedimentary rocks are formed from the accumulation of sediments, like sand, mud, or the remains of organisms. These sediments are often deposited in layers, one on top of the other. Over time, these layers are compressed and cemented together, forming sedimentary rock.

* Igneous rocks are formed from cooled magma or lava, and typically have a more crystalline or glassy texture.

* Metamorphic rocks are formed when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are transformed by heat, pressure, or chemical reactions. While they can sometimes have layers, these layers are often formed by the re-arrangement of existing minerals rather than the accumulation of sediments.

So, if you see a rock with clear layers, it's a good indication that it's a sedimentary rock!