Two substances that sublime, or sublimate, are ice and carbon dioxide. Sublimation is a phenomenon where a substance converts directly from a solid into a gas without becoming a liquid in between.
This curious natural event occurs when a substance, such as carbon dioxide, reaches a certain temperature. All substances have a triple point, a specific temperature and a specific pressure at which they can exist at all three phases, solid, liquid and gas, at the same time. If a substance has not reached the proper pressure to transform into a liquid, but has reached the proper temperature at which it can transform into a gas, it transitions from a solid to a gas directly without melting first. With carbon dioxide, this can occur at room temperature. With ice, this can occur at high altitudes where air pressure is less than at sea level, but only under conditions of very low humidity and very strong sunlight or heat.