A container that is capable of holding contents at temperatures above or below ambient for long periods of time due to vacuum insulation. The dewar flask consists of two containers inserted into each other and joining at the neck, the space between them being evacuated. Since a vacuum essentially conducts no heat, the content of the dewar flask is thus well insulated thermally. Vacuum flasks or thermos are commonly used to store hot or cold beverages. Large dewar flasks are often used to store liquid nitrogen and to freeze biological samples.