Crude oil is a fossil fuel extracted from the ground. Much like making strong alcohols, the crude oil is sent through a process called fractional distillation to make diesel fuel. First, the oil is heated by steam, which vaporizes most of it. It is them transferred to a distillation column, which allows the vaporized oil to condense as it rises and cools. The heavier parts of the oil will settle to the bottom, while rise and form into liquids. The separated parts of the oil can now be processed with various chemicals to produce diesel fuel.