The controlled process by which hydrocarbons are broken down or re-arranged into smaller, more useful molecules is called Cracking. The process allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules. Cracking may be done using heat energy (thermal cracking) or using a catalyst (catalytic cracking). Fractions containing large hydrocarbon molecules are vaporized and passed over a hot catalyst, thus breaking chemical bonds in the molecules and forms smaller hydrocarbon molecules.