Materials can generally exist in three states: solid, liquid and gas - you can change from one to another by altering temperature and/or pressure. But if you increase temperature AND pressure, the distinction between liquid and gas eventually disappears - this occurs at the "critical point". Above this, carbon dioxide behaves partly like liquid, and partly like gas. It spreads out like a gas to fill the available space, but can still dissolve substances as if it was a liquid. For carbon dioxide, this occurs above 72.9 atmospheres of pressure and 31.1 degrees Centigrade. The low temperature make supercritical carbon dioxide particularly convenient, since it can be used with substances that would be damaged by the high temperatures required to reach supercriticality for other materials. Dr Kurt Zozel, a Swiss scientist, discovered it could be used to remove caffeine from tea and coffee, and this process is now used in preference to the original solvent, methylene chloride, a suspected carcinogen. Other uses of supercritical carbon dioxide include the extraction of hops, pharmaceuticals, essential oils, and environmental pollutants. Technologies are also emerging in dry cleaning and paint spraying.