Thermos was the inventor of the type of insulated flask. The concept of these flasks is that they are two flasks, one inside the other. Between these flasks there is a layer of air or other gas. The inner flask will quickly reach the same temperature as the liquid but it will take a long time to change the temperature of the layer of air and then the outside flask. This means that the liquid inside stays the same tempearture for longer. Kleen Canteen is a competitor to Thermos and provide similar flasks. But there are plenty of other manufacturers and brands out there. Of course they all have different models which have different advantages and disadvantages to them.
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