Inside a refinery tower, the liquids and vapors separate into components or fractions according to their density and boiling point. The lightest fractions, including gasoline and liquid petroleum gas vaporize and rise to the top of the tower, where they condense back to liquids. Medium-weight liquids, including kerosene and diesel oil, stay in the middle. Heavier liquids, such as gas oils, separate lower down. The heaviest fractions with the highest boiling points, such as tar-like fractions called residue settle at the bottom.