The thermal conductivity of glass is not very good, so one side of thick glass in contact with hot water expands quickly when heated, while the other side has not expanded, which easily leads to the grain boundary slip of glass crystals. If the grain boundary slip exceeds the critical value, the glass will crack.

Therefore, if the glass itself has a low temperature and is used to pour hot water that has just been boiled, it is easy to crack. Or just after filling boiling water, wash it with cold water immediately, which is also easy to crack.

Mainly depends on the temperature difference. Theoretically, the temperature difference between hot and cold of thick glass should not exceed 35 degrees Celsius.