|How does a vacuum flask operate? — MA, Altamonte Springs, FL
A vacuum or "Dewar" flask is a double-walled vessel with a vacuum between the two walls. It's effectively a bottle inside a bottle, with their only contact at their mouths. Since there is no air in between their bodies, no heat can flow from one bottle to the other by either conduction or convection. The only way in which heat can move between the bottles is via thermal radiation. By coating the surfaces of the two bottles with highly reflective materials such as aluminum, even thermal radiation can be almost prevented from transferring heat between the bottles. Since there is virtually no way for heat to flow into or out of the vacuum flask, except through its mouth, the flask can keep hot things hot or cold things cold for extremely long times.