When lava comes out of volcanoes, sometimes volcanic gases bubble through the lava. When this type of lava sets, you get pumice. Pumice is so light that it will easily float in water (see the proof on the right!) The Romans mixed pumice with their cement to make it lighter when they made the dome of the Pantheon in Rome.
Pumice is sometimes used to clean yourself in a bath, because it is scratchy, and it floats in the bathwater.
Larger pictures of Pumice:
A polished specimen (why anyone should want to polish Pumice - I don't know!).
Pumice has many holes in it from volcanic gases, so it floats.
