Here's why:

* Intrusive igneous rocks: Granite is an intrusive igneous rock, meaning it forms when molten rock (magma) cools and solidifies beneath the Earth's surface.

* Cross-cutting relationships: A fundamental principle in geology is that an intrusive rock is *always* younger than the rock it intrudes into. This is called the principle of cross-cutting relationships.

* Sequence of events: The limestone must have already been deposited and solidified before the magma could intrude into it.

Think of it like this: The limestone is a pre-existing layer of rock. The magma that forms the granite then pushes its way through the limestone, solidifying and creating the intrusive body.