Here's how it works:

* Superposition: In undisturbed rock layers, the oldest rocks are at the bottom, and the youngest rocks are at the top.

Since the granite is in contact with the sandstone and contains pieces of the sandstone, we can conclude:

* The sandstone is older than the granite.

This is because the granite must have formed *after* the sandstone was there, and the granite's formation included pieces of the sandstone.