Originally Posted by umarabbas

Dear, the flask will lose its heat energy to the surroudings through Conduction and Radiation (normally). For example: The flask losing energy to the table on which it is put is conduction and radiation is when it looses its energy to the surroundings.

We need a perfect insulator inside the flask. that's why we see that special spongy type material used in flask. they ensure that little heat is lost via conduction and radiation. the inner part is made shining so as to make the heat energy reflect within the flask. Shinny substances reflect most of the heat and absorb very little. A perfect example could be placing a mirror and a frying pan of steel in the sunlight. it will be observed that the mirror's temperature would be far lesser than the frying pan's temperature. Reason as i stated earlier is that the mirror absorbs less heat and reflects most of it...

i hope i was clear about it!