A thick glass tumbler cracks when hot water is poured in it. Why?

The glass comes in the insulator class of material which means it is a poor conductor of heat. When you pour a very hot liquid into a thick tumbler what happens is, the surface of the glass that comes in contact with the hot liquid heats up and expands as per its coefficient of thermal expansion, while the outer layer still remains cold and does not expands (because glass is a poor conductor and does not carry heat quickly). This leads to the development of thermal stresses which eventually cause cracking.