Here's why:

* Joints are fractures in rocks where there has been no movement along the fracture plane. They are caused by stress, usually from tectonic forces or cooling and contraction. Think of it like a piece of dried mud cracking.

* Faults are fractures in rocks where there *has* been movement along the fracture plane. This movement can be horizontal, vertical, or a combination of both.

Let me know if you'd like to learn more about joints or faults!