Why does food get hot in a microwave oven

Irradiation of food inside the microwave oven causes photon uptake. The energy liberated each time a photon is absorbed is not sufficient to cause bond breakage (as was the case with UV light) nor can these microwave photons cause excitation of electrons (which is why we see a colour during irradiation with visible light but not with microwaves). Again, the energy is insufficient to 

A microwave oven is more efficient than a conventional oven because it only heats the food without first heating the oven itself. 

Metals tend to reflect microwaves, hence the lightning seen when we put a metallic item in a microwave oven. The microwaves energy is not absorbed by the food, and is otherwise absorbed in order to ionize the (moist) air inside the oven. 

The food in a micro-wave oven is effectively steam-cooked because the heating occurs when water molecules receive the energy of the microwaves. 

The overwhelming majority of plastics do not contain water, nor do glass or ceramics, which is why they do not get hot when placed within a micro-wave oven. 

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