Question

Tea is put in a thermos flask. Why does it remain hot when it is opened after some time? (Good reason needed)

Answer 1

In a thermos flask the inside is double-walled with vacuum between the walls. thus the radiation, a method of heat transfer, is reduced by silvering the inner side of the outer wall and outer side of the inner wall. so if a hot liquid like u gave tea is stored in the flask the radiations from the hot inside wall are reflected back by the shiny outer wall across the vacuum. this keeps the tea hot.......
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Answer 2

Inside the flask there is a region of vacuum, around the space we put tea in. In vacuum heat is transmitted through only radiation, because there is no matter to carry heat across. The amount of radiation is further reduced by silver coating, which reflects radiation energy.

As the transmission or transfer of heat from inside the flask is blocked, tea is in a thermally insulated region or isolated region. So the heat energy in the Tea is retained over a long duration.

But still, due to small and gradual losses of heat over long duration, tea can become cold.