Here's why:

* Porosity: Sandstone has many spaces (pores) between the grains of sand that make it up. These pores allow water and other fluids to flow through the rock.

* Permeability: The spaces in sandstone are interconnected, which means the water can move through the rock. This is what makes sandstone permeable.

Other examples of permeable rocks include:

* Gravel: Similar to sandstone, gravel has large spaces between the stones.

* Limestone (fractured): While limestone itself can be quite dense, fractures and cracks created over time can make it permeable.

* Some Igneous Rocks: Rocks like basalt can be permeable due to the presence of vesicles (gas bubbles formed during cooling).

Important Note: Not all rocks are permeable. For example, granite is generally impermeable because its crystals are tightly packed together.