The origins of this word trace back to the Latin volare, which means “to fly.” In the 14th century, it was used to denote such winged creatures as birds and butterflies. Today it is often used to describe something that can explode or, especially in an astronomical context, an element or compound that vaporizes at a low temperature. Planetary scientists studying Mars, for example, use the noun volatile to describe a chemical such as water or carbon dioxide. In a similar vein, researchers studying how planets form around other stars call icy compounds (including water and frozen gases) volatiles to distinguish them from high-temperature rocky materials. In general usage—a “volatile stock market,” for instance—the word connotes violence and variability, the equivalent of TNT detonating or frozen carbon dioxide vaporizing rapidly and forcefully on the surface of a comet.