There is actually a vacuum between the food inside and the outside air. Since the vacuum allows there to be a lack of anything that would normally act as a conductor of the heat/cold, the temperature remains more contained and stable.
A thermos bottle has two walls between which there is an empty space. Both the inside and outside walls are usually made of materials that do not transfer heat well. Thus, the hot or cold contents of the thermos bottle stay hot or cold while the outer layer of the bottle and the air between the two layers slow down heat transfer either in or out of the bottle.
Many other mugs use a thick insulating barrier between the liquid and outside air/hands. Vacuum is one method of insulating, meanwhile coffee mugs and others use ceramic or a thicker substance for insulation.
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