Why do thermos bottles keep drinks hot or cold?
Short answer
Thermos bottles are built so that heat moves slowly between the drink and the outside air. This helps them keep drinks hot or cold for longer.
How it works
Many thermos bottles have double walls with very little air, or a near-vacuum, between them. With less air, heat has fewer opportunities to move by conduction and convection.
Some designs also use reflective inner surfaces to reduce heat transfer by radiation. The lid and narrow neck reduce heat exchange through the opening.
Key point
A thermos bottle does not create heat or cold. It slows the movement of heat. Because heat transfer is never completely stopped, the drink’s temperature gradually moves toward the temperature of its surroundings.