Why doesn't the metal ring on Campbell's Soup at Hand cause sparks in the Microwave?

[Duke_of_Rat]

*Originally posted by chorpler *
**One fun thing I’ve tried is sticking a wooden toothpick in a cracker or some other support structure to keep it upright, then lighting it on fire and putting it in the microwave. After a few seconds, you get wonderful violet-blue fireballs rising up off the flame. Oddly, though, it doesn’t seem to work with a plastic toothpick or a candle. Anybody know why this happens? What is it about a little flame, or a burning toothpick in particular, that creates these delightfully unnerving balls of electricity? **

I wouldn’t post this, but the OP has been pretty well answered.
We tried to dry a smoldering joint once, and got the same effect. The smoke really went off.