Why glass sometimes breaks

It is as difficult to precisely determine the strength of a light of glass as it is to accurately quantify the magnitude of loads which it will have to support during its lifetime. But, breakage will occur if the applied load exceeds the glass strength. A glass fracture starts at an origin point and grows, either slowly, or too rapidly (faster than the speed of sound in air) to be followed by eye. Close examination of fracture fragments at the origin can reveal much about the magnitude of excessive loads applied at the time of fracture and their resulting stresses. Understanding the cause of breakage then allows appropriate corrective measures to be applied to future similar glass designs where the probability of breakage can be made acceptably low and the consequences of any such breakage will be reasonable.