A device used to clean the internal surface of a pipeline. Pigs are usually barrel shaped, made of metal, and covered with metal brushes. They may also have rubber or plastic cups and be made entirely of plastic. They are inserted into the pipeline by means of a device called a pig-trap and pushed through the line by pressure of the flowing fluid, usually gas. The forward movement of the pig, together with its rotation, cleans the rust, liquids, and other undesired substances from the pipeline; also called a go-devil.