A thermos reduces the transfer of heat from its contents to its surroundings through the use of a vacuum, which surrounds the inner chamber. The same principle applies when a thermos is used for cold foods or beverages.
Heat transfer occurs via the processes of radiation, convection and conduction. The lid of the thermos limits convection. The design of a thermos uses a glass vacuum chamber encased in metal or plastic, which protects the fragile chamber. The vacuum chamber slows conduction and radiation.
Preparing the thermos for use by pre-filling with hot or cold water increases its efficiency. This step ensures the radiant and conductive warming or cooling of the materials surrounding the food and beverage chamber.