A vacuum flask is used to keep either fluids very hot, or icy cold for long periods of time. This is achieved by the prevention of heat passing between the inside of the flask and the outside world.
The flask consists of a double-walled container of steel or glass. These walls are reflective (usually silver in colour) so that the heat rays, similar to light rays, cannot enter or leave the flask. Between the walls is a vacuum, which alleviates the problem of heat conduction through the walls. All the other components of the flask for example, the stopper and support of the container, are made of insulating materials. These also assist in reducing conduction. A material such as cork can be used for this task.
