Technology
How does a thermos work?
A thermos works by blocking the three ways heat moves, keeping drinks hot or cold for hours. A vacuum between its double walls stops heat conduction and convection, while a shiny reflective lining bounces heat radiation back.
See it in motion.
Watch a 2-minute animated lesson that shows exactly how a thermos works.
Step by step
- 1It has a double wall with a vacuum gap between them.
- 2The vacuum blocks heat conduction and convection (no air to carry heat).
- 3A reflective (silvered) surface bounces back heat radiation.
- 4A tight, insulating cap limits heat escaping the top.
- 5The same design keeps cold drinks cold by keeping heat out.
Frequently asked questions
- How does a thermos keep drinks hot?
- A vacuum between its walls stops heat from conducting or convecting away, and a reflective lining bounces radiated heat back inside.
- Why does a thermos work for both hot and cold?
- It simply blocks heat flow in either direction — keeping heat in for hot drinks and keeping heat out for cold ones.
- Why is the inside of a thermos shiny?
- The reflective surface bounces infrared heat radiation back toward the contents instead of letting it escape through the walls.