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A vacuum (thermos) flask is designed to keep hot liquids hot and cold liquids cold. Essentially, we need to reduce the transfer of thermal energy by conduction, convection and radiation. The vacuum flask consists of a double-walled glass container, with a vacuum between the walls. Both walls are silvered on the vacuum side. The container is supported on foam plastic which is a poor conductor of thermal energy. No thermal energy can enter or leave the flask by conduction or convection across the vacuum. The inner silvered surface reflects radiation from hot fluids back into the flask. The outer silvered surface reflects radiation in the external surroundings away from the flask. The foam plastic support and the plastic cap also minimize the thermal energy transmitted by conduction through the thin glass walls of the flask. Lastly, the plastic cap stops convection and evaporation.