There are two main types of stills: pot stills and reflux stills. In a pot still, the pot (like a large kettle) holding the liquid is heated. The hot gas is cooled in a simple neck, or tube, and collected. Pot stills are used for making alcohol like whisky. A reflux still is much more complex. It works with a continual flow of liquid going into the boiler, and a continual flow of distilled liquid coming out. The distilled liquid can also be put through the process several times, which is why it is called a reflux still. This allows for a more pure product, and the person working the still can control the level of purity. Reflux stills are used in the chemical and petroleum industries.