Here's why:

* Light: Pumice is incredibly light, often floating on water due to its high gas content.

* Rough Texture: Pumice has a very rough, porous texture due to the numerous gas bubbles trapped within it.

* Hollow Spaces: The hollow spaces in pumice are the key to its light weight and rough texture. They form when volcanic lava cools quickly, trapping gas bubbles.

While other rocks can have some of these features, pumice is the most likely fit based on the combination of all three.