Give reason for the statement

A normal glass tumbler cracks when dipped in boiling water. Why?

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 then the surface of glass that comes in contact with the hot liquid heats up and expands, while the outer layer still remain cold and does not expands (because glass is a poor conductor and does not carry heat quickly). This leads to development of thermal stresses which eventually causes cracking.