Question

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

Answer 1

The reason for the tumbler cracking is:

1. The glass is a poor conductor of heat, so when very hot water is poured into a thick glass tumbler, the heat doesn't spread evenly between the glass's inner and outer layers.
2. This causes uneven expansion among the layers of glass, as in the outer layer slowly compared to the rapid expansion of the inner layer.

Hence due to uneven expansions, the thick tumbler cracks when hot water is poured into it.