Question

Why does a glass tumbler sometimes crack on pouring very hot tea?

Answer 1

because of sudden heating it break it also happens with me

Answer 2

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. This stress is sometimes too much and the glass cracks.