What are some good insulators for a thermos?

* Vacuum: A vacuum is the best insulator, which is why thermoses have a double-walled construction with a vacuum-sealed space between the two walls. This prevents heat from transferring from the hot liquid inside the thermos to the cold air outside.

* Air: Air is also a good insulator, which is why thermoses often have a layer of air trapped inside the walls. This layer of air helps to slow down the transfer of heat between the hot and cold sides of the thermos.

* Foams: Foams are made up of tiny bubbles of gas, which trap air and prevent heat from transferring. Thermoses often use a layer of foam insulation between the two walls.

* Reflective materials: Reflective materials, such as aluminum foil, can help to reflect heat back into the thermos. Thermoses often have a layer of reflective material on the inside of the outer wall.