Here's why:

* Pumice is a volcanic rock formed from frothy lava. The trapped gas bubbles within the rock create a very porous structure.

* Porosity means lots of air spaces. This makes pumice incredibly light, and it can even float on water.

* Density is mass per unit volume. The air pockets in pumice significantly reduce its overall mass compared to its volume, resulting in a very low density.

While other rocks might have a low density, pumice consistently takes the title of the least dense rock.