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Much of thermodynamics is about equilibrium, including the fact that two systems of different temperatures, when exposed to each other, eventually become the same temperature. If thermal equilibrium is desirable, we can expend our efforts to maximize the exposure of the two systems to each other. Conversely, in order to keep them from reaching a state of equilibrium, some sort of insulating barrier is required. Similar to the experience of using a thermos to keep coffee hot, insulators can slow down the temperature change but cannot stop it completely.