Ann Christensen, age 11, of Rowayton, Connecticut, for her question:

What exactly is pumice?

Its name is coined from an older word for foam, no doubt because this pale gray pumice stone looks like frozen foam. As a matter of fact, there was a,time when it was a thick foamy liquid. Pumice is an igneous rock, melded in the fiery fury of volcanic activity. It erupted from the bowels of the earth as a hot mass of molten minerals with more than its share of gases and steamy vapor. All hot lavas contain gases, but most of them float to the surface and escape as the mixture sets solid in the cooling air. Those that fail to escape leave pores in the solid rock.

The minerals in pumice stone are similar to those in solid granite. But its gaseous content makes a big difference, especially when these gases have a chance to expand as the mixture cools. The hot lava becomes a mass of bubbly foam. The bubbles remain trapped as the non gaseous ingredients cool and set around them. The solid pumice is riddled with airy pores and pockets    and all that trapped air makes it light enough to float on water.