Why does a glass tumbler crack when very hot milk is poured into it?

Cracking of glass tumbler on pouring very hot milk i because:

1. As we know glass is a bad conductor of heat.
2. On pouring very hot milk in a glass tumbler, it cracks because the inside of the tumbler expands quickly but the outside does not.
3. The unequal expansion of the glass tumbler results in cracking of the tumbler.

Thus, because of unequal expansion glass tumbler crack when very hot milk is poured into it.