I found this via GrrlScientist, whose posts are also on Science Blogs, a really cool place to browse.
This is Christmas lights. You can also cut a grape in half the long way and put the two halves cut-side-up on a paper plate, nearly touching each other, and turn the microwave on. The two halves will arc. Other fruits work, too, but it's been a long time since I did this, so I don't remember which ones did. Probably high-acid ones. I'd guess strawberries.
One thing they don't say is if anything was damaged. The bulbs arced inside each one, but it's not clear anything bad happened, either to the lights or the microwave. I gotta try this (though, to be responsible, I guess I should point out they say not to try it at home; yeah, right).