In the case of microwave ovens, the commonly used radio wave frequency is roughly 2,500 megahertz (2.5 gigahertz) .
Radio waves in this frequency range absorbed by water, fats, and sugars. When they are absorbed they are converted directly into atomic motion or vibration.
Therefore heavy motion or vibration is converted into heat.
Microwaves are not absorbed by most plastics, glass, or ceramics.
The frequency of Microwave equals the resonance frequency of water. Therefore foods could be heated up by oscillation of water molecules.