Here's why:

* Sediments: The small pieces of other rocks are called sediments. These can be sand grains, pebbles, clay particles, or even fragments of shells or fossils.

* Deposition: Over time, these sediments are deposited in layers, often at the bottom of a body of water.

* Cementation: Dissolved minerals in the water seep into the spaces between the sediments. As the water evaporates, the minerals crystallize and act like glue, binding the sediments together.

There are many types of sedimentary rocks, depending on the type of sediment and how it was formed. Some common examples include:

* Sandstone: Formed from sand grains cemented together.

* Conglomerate: Formed from larger pebbles cemented together.

* Shale: Formed from very fine clay particles.

* Limestone: Often formed from the shells of marine organisms.

Let me know if you have any more questions about rocks!