The Physics 2000 site someone posted here has a great entry-level primer on Microwave ovens.

http://www.colorado.edu/physics/2000...ves/index.html

I have one question that wasn't covered in it though. Why do the microwaves only affect water molecules? Are any other substances excited by them?

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...And that, my liege, is how we know the Earth to be banana-shaped. --Sir Bedevere