The refinery of oil entails the break down of crude oil down into its various components, which then are selectively changed into new products. All refineries perform three basic steps: separation (fractional distillation), conversion (cracking and rearranging the molecules) and treatment. The crude oil separates into fractions in a distillation tower according to weight and boiling point with the lightest fractions, including petrol and liquid petroleum gas (LPG), vaporize and rise to the top of the tower. Kerosene (aviation fuel) and diesel oil, stay in the middle of the tower whilst heavier liquids separate lower down with the heaviest fractions with the highest boiling points settling at the very bottom