Why does metal produce strange effects in a microwave?
Why does metal catch on fire in the microwave?
Come now. Metal — at least the kind you're likely to put in a microwave — doesn't catch fire. What you do see is sparks, which may pit the metal. You get sparks because the microwave radiation generates a fluctuating current in the metal, a process known as induction. Due to internal resistance in the metal, at any given instant there are sharp differences in electrical potential between different points on the metal surface, and sparks jump the gap. Basically tiny lightning bolts, these sparks are intensely hot and I suppose could set fire to, say, a paper plate. The metal may also become hot and incandesce (glow) or even vaporize.
That's not the real reason not to put metal in a microwave, though. The metal may reflect the microwaves back where they came from, namely the oven's magnetron tube. That could damage or destroy the tube, so needless to say you shouldn't try this at home.
Sometimes I sure wish I could, though. One hears reports of microwave experiments that quicken the heart of one's inner adolescent. A couple spuds who obviously got turned down for one Friday night date too many have written to say microwaving a filter cigarette will cause flame to shoot out the filter end. Nuking a marshmallow supposedly also produces cool results. I wouldn't know — Mrs. Adams has been adamant about not risking the household microwave in the interest of science. Thank God she wasn't married to Edison, or we'd all be sitting in the dark.