How does a stainless steel thermos flask stay hot?

I was mostly just wondering, dtailed answers are appreciated. But as always, all answers are appreciated.
Once I had hot water in a stainless steel thermos flask and it was still hot 2 days later, I was wondering why.

it’s because it’s a vacuum flask, you have two stainless steel flasks welded together with only a vacuum between, a vacuum is a very poor conductor of heat so it’s a very good insulator

http://home.howstuffworks.com/thermos2.htm

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