Man, I did that in my Video Production class, in the TEACHER's microwave...hahaha..(we DID ask him, and he had a confused look on his face, but said it was ok to put cds in the microwave) anyways, after they spark and crack an all that, smoke starts pouring out of the microwave while the door is still shut, and scares the crap out of you. starts smelling like crap (really
really strong smelling crap) so you stop it, open the door and run outside. We did it several times

We also did it with grapes...damn thoise are cool! Cut a grape (might have to be green grapes) lengthwise but leave a strip of "skin" attached at the bottom of the cut, and stick it in face up (flat sides up) but still attached in the microwave, set it for 25 seconds, and sparks galore...also smells like crap