An igneous intrusion is younger than the rocks it intrudes into because of the very nature of how it forms:

* Igneous intrusions form from molten rock (magma): Magma rises from deep within the Earth, pushing its way through existing rock layers.

* Intrusive cooling: The magma cools and solidifies within the pre-existing rock layers, becoming an igneous intrusion.

* Principle of Cross-Cutting Relationships: This geologic principle states that any feature (like an igneous intrusion) that cuts across another feature (like existing rock layers) must be younger.

In simpler terms:

Imagine you're making a cake. You've got the cake batter (representing the pre-existing rock layers) all poured in the pan. Then you add frosting (representing the magma) to the top. The frosting is obviously added after the cake batter, so it's younger. The same concept applies to igneous intrusions!