Why will a thick glass tumbler break when hot water is put into it ? Why will the outer surface crack ?

Glass, like many substances expands on heating. On pouring hot water in a glass tumbler, its inside expands due to the heat of the water. However, since glass is a poor conductor of heat, this heat can not transferred to the outer surface of the glass. This means that from inside  glass is expanding but not from  outside. This uneven expansion causes stretching and hence the outer surface cracks.