The first step in making dry ice is to turn carbon dioxide gas into a liquid by compressing the CO₂ and removing any excess heat. The CO₂ gas is further cooled until it condenses into a liquid. Next the pressure is reduced over the liquid carbon dioxide by sending it through an expansion valve. Part of the liquid sublimates (changes directly from a solid to a gas), causing the remainder to freeze into snowflakes. The dry ice is then compacted together under a large press to form blocks or pellets.