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How to Heat Up a Thermos Flask

Updated April 17, 2017

Thermos flasks are great to carry warm drinks in, especially in winter, because the insulation in the Thermos helps to keep the liquids warm for longer than a normal container would. For optimal results, ipreheat the Thermos flask before filling it with the liquid that you want to keep warm.

Boil a pot of water.

Carefully fill the Thermos flask with the boiling water; if the Thermos flask has a narrow opening, you may want to use a funnel to safely direct the water into the Thermos without spilling.

Place the cap onto the Thermos flask and let it sit for about five minutes. The purpose of this is to let the boiling water heat up the metal insides of the Thermos.

While you are waiting for the boiling water to heat the Thermos flask, warm up the liquid or food contents that you will be placing inside.

Remove the top from the Thermos flask and pour out the boiling water after five minutes have passed.

Carefully pour the liquid or food into the Thermos, using the funnel if needed.

Things You'll Need

  • Pot
  • Boiling water
  • Funnel (optional)
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About the Author

Charli Skipper began writing professionally in 2002. She has written articles for "Tiger Weekly" and is now providing technical writing for state government. Skipper holds a Bachelor of Arts in theater from Louisiana State University and a Juris Doctor from the Paul M. Hebert Law Center.