Question

Why do a normal glass tumbler cracks when dipped in a boiling water

Answer 1

The crack is caused by differential thermal expansion. The part of the glass that the boiling water touches first expands due to the heat, but the heat doesn't conduct through the glass quickly so there is stress between the expanded inside of the glass and unexpanded outside of the glass.

Answer 2

W hot liquid is poured into a glass tumbler or normal it cracks the reason is that glass is a poor conductor of heat when hot liquid is poured into a tumbler the inner surface of tumbler becomes hot while its outer surface remains at the room temperature therefore the inner surface of tumbler expands while its outer surface does not expand than equal expansion cracks tumbler