Why does a clinical thermometer break when kept in boiling water?

Glass is a poor conductor of heat and expands on heating.  When a thermometer is placed in boiling water, the mercury is heated up and it in turn heats the inner portion of the glass.  The layer of glass is thick and as it is a poor conductor of heat, the outer portions of the thermometer are not heated. Therefore, the inside layer of glass expands but not the outer layer and hence it breaks.