Probably it is about the fact that water is polar and readily absorbs microwaves. Which, of course, is how microwave ovens work. (The standard 2.4 GHz frequency

Probably it is about the fact that water is polar and readily absorbs microwaves. Which, of course, is how microwave ovens work. (The standard 2.4 GHz frequency is actually not absorbed as well as higher freqs; this is a deliberate choice to let the radiation penetrate deeper into the food and heat it more evenly.)