Trans fat is a type of unsaturated fat. Trans fat is often produced in higher amounts in factories when manufacturing certain processed foods such as margarine. Trans fat occurs in nature only very rarely, found in small amounts in meat and milk fat. When vegetable oils are heated or when they are "hydrogenated", trans fats are made. Hydrogenation is the process of bubbling hydrogen gas through the oil to change its consistency. The bubbling raises the melting point of the oil. As the hydrogen passes through, the oil begins to become solid. By stopping the hydrogenation part of the way through, manufacturers obtain "partially hydrogenated oil". This is similar to butter, but much cheaper to produce. It is sold as "margarine", "oleo" or "vegetable shortening". The process allows cheaply adding a butter-like consistency to food products.