"Dipole rotation is the mechanism normally referred to as dielectric heating, and is most widely observable in the microwave oven where it operates most effecti

"Dipole rotation is the mechanism normally referred to as dielectric heating, and is most widely observable in the microwave oven where it operates most effectively on liquid water, and also, but much less so, on fats and sugars.": https://en.m.wikipedia.org/wiki/Dielectric_heating https://en.m.wikipedia.org/wiki/Dielectric_heating In ice the molecules are not free to rotate as they are in liquid water.