On a molecular level, when a metal such as steel cools down rapidly, the crystalline grains in the metal transform into a lenticular shape. This causes the metal to become harder but also more brittle in the process. Heating the metal to an appropriate temperature causes a homogenous growth of new crystals. The more heat is applied to the metal, the more its ductility increases and its hardness decreases. During annealing, the steel is heated above its critical temperature and held at this temperature for a period of time before controlled cooling. The controlled cooling process allows the crystals to diffuse into a more flexible, cubic-shaped state.