Heat can be transferred from one place to another by three methods: conduction in solids, convection of fluids (liquids or gases), and radiation through anything that will allow radiation to pass. The method used to transfer heat is usually the one that is the most efficient. If there is a temperature difference in a system, heat will always move from higher to lower temperatures.
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There is more than one way to heat water, so I would not apply this to all situations, but in general, when you heat water you create convection currents which cause the hotter water to mix in with the colder water.