Rocks are solid at room temperature. They are made of grains that fit together. Each grain in a piece of rock is made from a mineral, which is a chemical compound. The grains in a rock can have different:
Some types of rock have interlocking grains that fit tightly together. Granite is a rock with interlocking grains. Other types of rock have rounded grains. Sandstone is a rock with rounded grains.
Rocks with rounded grains are more likely to absorb water than rocks with interlocking grains. This is because the water can get into the gaps between the grains. Rocks that absorb water are described as being porous.
Rocks with rounded grains are usually softer and more crumbly than rocks with interlocking grains. So porous rocks tend to be softer than non-porous rocks.