Here's the breakdown:

* Formation: Pumice is a volcanic rock formed when lava erupts with a high gas content. As the lava cools rapidly, the gas bubbles get trapped inside the rock.

* Density: These trapped air bubbles make the pumice very porous and light. The density of pumice is less than that of water, making it float.

* Example: Think of a sponge – it's full of air pockets and floats because of its low density. Pumice is similar, but it's made of solidified lava instead of a squishy material.

So, while the rock itself is solid, the trapped air bubbles create a lower overall density, allowing the pumice to float on water.