Tempering is a thermal or chemical treatment that compresses the surface layer of the glass, thus improving its resistance to breaking. An 8 mm-thick tempered sheet resists breakage from a drop of a 1 kg marble free falling from a height of 2 meters. The same marble, breaks an annealed sheet of the same thickness. When tempered glass breaks, the stored energy is freed in the form of surface energy; for this reason, when it breaks, small blunted fragments are created, unlike when ordinary glass breaks; thereby, the importance of tempered glass as safety glass. Consequently, any attempt to cut, puncture, or model the tempered glass leads to its breakage. All these operations must be carried out before tempering.