Here's why:

* Mud: Mud is a mixture of fine-grained clay and silt particles.

* Compaction: Over time, the weight of overlying sediments compresses the mud, squeezing out water and causing the particles to bond together.

* Shale: This process creates a fine-grained, layered sedimentary rock known as shale.

Shale is often very brittle and can easily break into thin pieces. It is also a common source rock for oil and natural gas.