Question

A thick glass tumbler crack when hot tea in poured into it.

Answer 1

A thick glass tumbler crack when hot tea in poured into it as the molecules should undergo the sudden change in temperature and the thermal expansion will be more vigorous resulting in cracking of the glass

but if we pour slowly the heat gets transferred in a well defined manner and the glass wont break 

Answer 2

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 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 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.