Porosity (or void fraction) is the fraction of the volume of some solid material that is void, i.e., it is the volume of the empty space within holes and tunnels within the material. Some rocks found on Earth are a bit honeycombed (porous) and a liquid such as water would flow right through. Porosity is of interest in planetary science both as an aspect of a planet or moon's current structure, and because of porosity's influence on the processes of planet formation and evolution include cooling times. The porosity of the material on the Moon and on Mars are of such interest. Measurement of porosity is a challenge. Porosity is reflected in the density, and gravimetry offers clues to that.