An important concept when talking about a thermos is heat. Heat means more than just what's hot and cold. Heat can be quantified as the amount of thermal energy a material has - basically, how fast the temperature is causing the molecules to wiggle. The faster they wiggle, the hotter the material is.

When heat is transferred from one object to another, the wiggling molecules transfer their energy to the molecules of the other material. Whether something is heating up or cooling down, it has to be able to give or get thermal energy. But thermoses are made out of specialized materials, called thermal insulators, that have been found to not transfer this energy very well.