Why do microwaves only heat up polar molecules?

An apolar molecule will not try to allign the dipole, hence microwaving it might not work. I write might, because you might match exactly the radiation to induce some quantized transitions of the rotation of the molecules, which would eventually heat up the system. read more

1- Water molecules will start oscillating, the dipole moving with the field. 2- The oscillation of the field is “slow” enough for the molecules to follow the oscillation but “fast” enough that the oscillation will be converted int appreciable heat in enough time (with a radiowave it would take decades perhaps!). read more

Microwave is acting by switching the electric field, so that molecules that have dipole moment such as water start vibrating, moreover hydrogen bonds between water molecules break and when they reform heat evolving is supplied to surrounding molecules which causes the whole dish to heat up. read more

The conclusion is that earthenware ceramics heat up in the microwave because they have the polar water molecules in them which undergo dielectric heating. On the other hand, stoneware (and apparently porcelain) will not heat in the microwave due to their respective lack of water molecules. read more