Pumice is a type of volcanic rock so light it can float on water. When a volcano erupts, lava filled with gas bubbles can cool so quickly that the bubbles get trapped inside, creating a rock full of tiny holes like a sponge made of stone. These air pockets make pumice incredibly lightweight: a chunk the size of your fist might weigh less than an apple.

People have used pumice for thousands of years because of its rough, scratchy texture. The ancient Romans ground it into powder for toothpaste and used blocks of it to smooth stone buildings. Today you might find a pumice stone in your bathroom: people rub these gray, porous rocks against rough skin on their feet to make it smoother. Pumice is also ground up and added to some heavy-duty hand soaps to help scrub away stubborn dirt and grease.

The rock forms most commonly from volcanoes that produce light-colored, silica-rich lava. Major pumice-producing volcanoes exist in places like Italy, Greece, and around the Pacific Ocean's “Ring of Fire.”