Vacuum Insulation: The flask is composed of two bottles, one placed inside the other, joined only at the neck. The air is removed from the space between these two layers during manufacturing, creating a near-vacuum. Since heat requires molecules to travel through (via conduction or convection), the vacuum effectively stops heat from escaping or entering.
Reflective Surface: The inner wall of the flask is often coated with a reflective material, such as silver, to reflect radiant heat back into the liquid, further reducing heat loss.
Airtight Cap: A tight stopper or lid is crucial to prevent convection, which is the loss of heat through escaping steam or the intake of cooler air.