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Glass is used in a thermos flask. Glass is an insulator and obviously a very bad conductor of heat. Hence heat is never lost by conduction in a thermos flask. Hence things are always kept hot.
Thermo flask is like a bottle inside another bottle and sealed the ends by melting inside. the air is removed out from the bottles to produce a vaccum inside,which is not a good conductor and does a good job at slowing down the movement of heat andhence the liquid kept hot.