Tempered glass ("heat-strengthened", "toughened glass") is plate glass that has been heat tempered to increase its strength. Glass heated to its softening point floats on a bed of tin (which is why it is sometimes referred to as "float" glass or "safety float" glass), and air jets blast the surface of the glass with cold air. This causes the glass to harden quickly on the outside, while the inside continues to cool slowly and to contract. This contraction puts the outer layers in compression while the inside of the glass is in tension. Because of this process, tempered glass is five to eight times stronger than ordinary glass. Even it break, it shatters, the fracture edge is round not sharp which is also relatively safe for the people.