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How Thermoses (Vacuum Flasks) Work

Most people have or are familiar with the "Thermos" (also known as a vacuum flask or a dewar). I can remember as a kid having one that came with my lunch box. One day my mother might put grape juice in it and at lunch I would have nice, cold grape juice. The next day she would put hot soup in it and I would have hot soup for lunch. And I can remember asking, "How does it know whether to keep stuff hot or cold?" Where's the switch, in other words...

Or, similarly, "You heat things up in an oven and cool them down in a refrigerator -- how come this thing can do both?" In this edition of HowStuffWorks, we will learn how a Thermos "knows" what to do.