when mary took a cold thick-walled glass out of the refrigerator and placed it in boiling water, the glass cracked. Explain why

The outer surface of the glass expands faster than the inside surface since glass is a fairly good thermal insulator. The outer surface tries to expand without the inner surface doing so at the same time. This is the secret of Pyrex glass. It is more conductive to heat and the inner and outer surface expand (and contract) at the same time.