One theory:

"Our suspicion is that the grape halves act as a small dipole antenna for the microwaves. Optimally such an antenna is 1/4 the size of the wavelength, which is pretty close for our grape. The skin flap is a conductor and currents run back and forth across it. As it heats up, it dries out, increasing it's resistance further, causing more heat, etc. Finally it ignites. The smell of burnt sugar fills the oven after it is turned off. The spark will not occur if the grape is too wet. Juice boils up and covers the skin bridge and there is no spark. On rare occasion we noticed rising clouds of luminous plasma (often referred to as 'ball lightning' in other microwave oven web pages."

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Really, at this point, shouldn't Corelle's Golden Butterfly pattern be adopted as Slashfood's official plate mascot?

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We really had the same boring youth! But unlike you, I did tons of experiments with my parent's microwave ovens. You don't need to find the thermal hot spot of the microwave though... just place your grape, sliced in two leaving just a tiny membrane attached and microwave them for a few seconds. It works 75% of the time.

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we should do a poll to see how many people actually STILL have the golden butterfly, and give a prize to the person who has the most pieces, including casseroles, etc. LOL!!!!

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this is gonna sound kinda stupid, but i swear i was one of the first people to cut a grape that way to get the plasma. many people used to cut the grap just in half, but cutting the half in half seems to work better. anyway, here's a link to some videos some colleagues and i did a couple years ago.
http://home.earthlink.net/~marutgers/fun/microwave/microwave.html

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