Why do thin glass tumbler break when filled with very hot water?

Glass tumbler break due to the difference in thermal expansion of their inner and outer surfaces.Glass is known as a bad conductor of heat.Hence when the tumbler is suddenly filled with hot water,the inner layer of the glass gets heated immediately as it comes into contact with hot liquid.But the outer layer remains at a lower temperature for some more time as the heat is conducted through the glass only slowly.

As a result,the inner layer tends to expand more than the outer layer.This leads to cracking.Thick-walled tumblers and tumblers made of special quality glass withstand such cracking to a certain extent.