A thermos or vacuum flask is used to keep the temperature of its contents constant. Thus if we put hot water into the flask,it will stay hot and if we pour ice-cold water into the flask,it will remain ice-cold.
The essential features of a vacuum flask consist of a double walled glass vessel with the insides of the walls coated with silver which prevents heat loss by radiation (Silver is a poor radiator and any heat radiated from one wall tends to be reflected back by the other) and a vacuum between the walls which prevents heat losses by conduction and convection, since a material medium is required for these two processes.
The stopper is made of an insulating material such as cork or plastic and the bottom of the glass vessel is supported with insulating cork.
The insulating cork or plastic stopper reduces heat loss by convection from the hot contents upwards to the outside.