Microwaves are absorbed by wires creating current in the wires which can drive a neon lamp like this guy http://en.wikipedia.org/wiki/File:Neon_lamp.jpg. I drilled a grid into a piece of plastic and slipped in the bulbs, leaving the wires to hang out like antennas. As the plate turns, the bulbs go into and out of places where the microwave energy is denser, illuminating the bulbs. No, it doesn't seem to hurt the microwave. You can find out more about how to make this by going to http://kossover.squarespace.com/journal/2010/11/12/seeing-where-the-microwaves-are.html.
Edits: I do seem to suggest that microwaves are only absorbed* by things that conduct electricity. That's not true, of course. The changing electromagnetic field of the microwaves will oscillate anything that has a charge or is polar. I was trying to point out that microwaves can move electrons in wires. Even slightly salty water does seem to heat faster in my poorly controlled experiments, though.
"Absorbed" isn't quite the correct term, I know.
Oh, and kids, don't try this at home without your parent's permission. Many things do not like going into a microwave and will catch fire or spark or both. Your parents will be mad, and the fire department will be annoyed as well.
Marc "Zeke" Kossover, physics and environmental science teacher, The Jewish Community High School of the Bay, San Francisco, California
