The Different Types of Microwave Ovens

Microwaves have about the right energy to excite vibrations in molecules. In particular water molecules are very useful because they are dipoles. Water has a very high specific heat capacity, so although it's heated effectively by microwaves, you also have to put in a lot of energy to just heat it up. It's not as effective in carbohydrates, but it's mostly non-polly and a big long chain of carbons. And it will still heat those to some extent. So foods with high water content will generally be microwaved more efficiently than foods with a low water content.