Why does the glass shatter by itself?

The glass in your home may be tempered glass. Only tempered glass will produce burst fragments.
When tempered glass is manufactured, due to some factors, cold and heat are not uniform on the assembly line, the glass itself generates internal prestress when it cools. This prestress is under a certain suitable condition (temperature, humidity, vibration, etc.) will be released, resulting in the shattering of the glass.
When the tempered glass is broken, the fragments are generally uniform, and the edges and corners are not very sharp, so it rarely hurts people.
The reason why the glass burst is because of expansion. As soon as hot water is poured into the glass, the inner wall of the glass is heated rapidly.
Although the outer wall of the cup remains the same, the inner glass is suddenly squeezed outward, and the cup is broken. If the inner and outer sides of the cup are heated at the same time in advance, and then hot water is poured in, the difference in the degree of expansion between the inside and outside of the cup is not so big. , it will not burst. However, if the glass is very thin, even if you pour hot water, the heat will quickly spread to the outside, so that the inside and outside will expand at the same time, and the glass will not be easily broken. In addition, the so-called hard glass and resistant Hot glass, because the proportion of expansion is small, it is not easy to burst.