It is made of two glass bottles inside and outside. The two are connected at the mouth of the bottle. The gap between the two bottles is vacuumed to weaken the convection. The surface of the glass wall is coated with bright silver film to reflect the infrared radiation. At high temperature inside the bottle, the thermal energy of the contents does not radiate outward. When the temperature inside the bottle is low, the heat energy outside the bottle does not radiate into the bottle.