[pfedak]

To set the record straight for the above comment -

- it's true that there isn't a precise frequency needed for microwave ovens to heat food

- however, "polarity flipping" is just a description of electromagnetic radiation itself, and shooting enough EM radiation at food in a frequency range it absorbs will heat it up via dielectric heating

- microwaves have no relationship to any specific resonant frequency of water - the vibration frequencies are orders of magnitude higher https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_... while rotation response inherently does not have a peak

- otherwise, yes, the motion of (polar) molecules induced by an electric field is indeed the mechanism of dielectric heating