My understanding of microwaves was that due to the high frequency they are easily absorbed into any material. The more "loose" the material, the easier the absorption.
I know it's dangerous to let microwave on when empty, mostly because there is nothing that will absorb the microwaves and that may cause magnetic "storm" inside :) But I heard today that it's equally dangerous to use microwave to preheat plates.
I would like to know if and why it's true. I think that the plate is a material like any else. But due to the high density it absorb the microwaves slower than say a glass of milk. And also the vibrations may cause it to break after extended period of time.