Sublimation is what happens when a solid turns directly into a gas without becoming a liquid first. This might sound impossible, since we usually see ice melt into water before it evaporates, but sublimation skips the liquid step entirely.

Dry ice (frozen carbon dioxide) is the most dramatic example. Leave a chunk of dry ice on a table and it seems to disappear as it transforms straight from solid to gas. Snow can also sublimate on cold, dry days: the snow vanishes even though it never melted into puddles. This is why frozen foods in your freezer sometimes develop freezer burn. Ice crystals on the food's surface sublimate, leaving the food dried out.

The opposite process, when gas turns directly into solid, is called deposition. Frost forms through deposition: water vapor in the air becomes ice crystals without ever being liquid water.

Scientists use sublimation as a purification technique, heating substances until the pure material sublimates away from impurities. Understanding sublimation helps explain everything from how comets lose mass in space to why that forgotten ice cube tray in the back of your freezer holds smaller ice cubes than it used to.