* Faults are breaks in the Earth's crust: Faults form when stress on the Earth's crust causes rocks to fracture and move past each other.

* Rocks must exist before they can break: For a fault to form, there must be existing rocks for it to cut through. The rocks must already be in place to be subjected to the stresses that create the fault.

* Faults disrupt existing rock layers: When a fault forms, it disrupts the original layering and continuity of the rocks. This disruption is a clear indication that the fault occurred after the rocks were formed.

* Relative dating: Geologists use the principle of cross-cutting relationships to determine the relative age of geological features. If one feature cuts through another, the cutting feature is younger. In this case, the fault cuts through the rocks, meaning it must have formed after the rocks were formed.

In summary:

Think of it like this: you can't break a cookie before it exists. Similarly, you can't create a fault in rock that hasn't been formed yet. The rocks must be there first, and then the fault can form and cut through them.