The crude oil is a mixture of lots of different oily substances. Fractional distillation splits crude oil into separate fractions.

This happens in a distillation tower (which we shorten to still). The different fractions come out of the still at different levels. The gases come out at the top, the runny liquids in the middle and the thick, sticky liquids at the bottom.

The crude oil is heated in the bottom of the still so that it all boils.

The thick, sticky oils don't get far up the tower. They condense back into liquids near the bottom - even though it's still quite hot.

The thinner, runny ones get all the way to the top of the tower before they condense.

The condensed liquid is collected and piped out of the still.