But microwaves don’t heat water evenly, so the boiling process is difficult to control. Microwave ovens shoot tiny waves into the liquid at random locations, causing the water molecules at those points to vibrate rapidly. If the water isn’t heated for long enough, the result is isolated pockets of very hot or boiling water amid a larger body of water that’s cooler. Such water may misleadingly exhibit signs of boiling despite not being a uniform 212 degrees. For instance, what appears to be steam rising from a mug of microwaved water is only moist vapor evaporating off the water’s surface and condensing into mist on contact with cooler air—it’s the same principle that makes our breath visible on frigid days.
via Microwaving water for tea: Why are the results so lousy?.
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