The Vacuum
The first principle to understand in the workings of a Thermos flask is the concept of a vacuum. A vacuum is defined by its having absolutely nothing: no air, no particles, no "anything". This is the idea behind the Thermos. Because it has "nothing", a vacuum has a few useful properties in regards to thermodynamics. For one, it can't transfer heat in either direction. Because there's no matter to transfer the heat through, heat will remain in place when confronted with a vacuum.
The Vacuum of the Thermos
For a Thermos flask, the vacuum used isn't technically a complete vacuum. Because it is difficult to create a vacuum on a large industrial scale, the vacuum used in the Thermos flask is merely an area with an atmospheric pressure that is much lower than the air around it, which accomplishes the same job. This lack of air and particles in the "vacuum" area allows for minimal heat transfer in a Thermos flask.
How it Works
The Thermos flask is usually made of either metal or plastic. It's constructed so there's a hollow space in between an inner and outer wall. In this hollow space, a vacuum is created when the air is sucked out. After this, the hollow space is sealed to maintain the vacuum.
When a liquid is poured into the Thermos flask and then the top is sealed, the liquid inside maintains its temperature because heat does not transfer through the vacuum. No heat can enter the Thermos to warm a cold liquid, and no heat can leave to cool a hot liquid.
Tags: Thermos flask, hollow space, this hollow, this hollow space, transfer heat, vacuum used
