A liquid has particles that aren't held so tightly together, so they can move around relative to one another . Warm them up, give them energy, and they whizz around each other much more quickly. This increase in energy in turn makes the temperature of the food increase. Since the food is transparent to microwaves, the waves get right to every part of the food at once, so cooking times are dramatically reduced. The point is that a liquid's particles can be moved much more easily by microwaves than the firmly-fixed particles of a solid.
Liquids therefore cook much more qickly.
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