Gas was a state of matter that expanded to fill a space. Gases could be invisible, such as oxygen, or could appear as a visible, misty substance, such as Clouzon-36. Many terrestrial planets had an atmosphere, a collection of gases that formed a relatively thin veil over the world's surface, while gas giants were composed almost entirely of gas. Gas mines such as Bespin's Cloud City were used to extract and purify gases from a planet's atmosphere. Most living organisms required gas to breathe, while some gases were poisonous to certain species. Some gases could also be used as drugs, such as to render individuals unconscious. Most living beings could also produce gasses through flatulence (informally known as "farting"), as flatulence was the accumulation of gasses in the digestive system which usually needed to be expelled. Unnaturally high levels of flatulence could be caused by incredibly potent spices, such as anilam, which almost ruined Snap and Karé's wedding when it caused all who had consumed the spice to release gas uncontrollably.