I saw my mother washing thermos flask with warm water. I ask to her why she do so. She replied: " because if you wash it with cold water, then may it get broken." I wondered how but she had no reply. Rather, she promissess that she had such experiences. This was my problem. Why the glass inside thermos flask get broken when treat with cold water? ( cold water means not so cold. I mean at $27^0$ ). I have no well knowledge about the properties of materials used to make thermos flask, but my problem is that, if it can work well in hot, then how it get broken when subjected to cold water?
I search it in web and there shows manual of some thermos flasks tell that to use warm water to wash it and avoid cold water, as my mother said.