Canoe -
Microwave safe plastic I would guess blocks some of the waves, and aluminum foil blocks pretty much all of it... and turns it directly into heat. This heat will first cause flaming pieces of the metal to fly off (sparks) but a little while later will cause it to rapidly oxidize (burn) and turn into a light gray powder.
As to it effecting the human body.... as canoe said, it excites the molecules, which basically means it heats them up. This is ok for a bit with the human body, but if you get a molecule excited next to DNA... that could cause cancer. Probably won't, and you'd probably need to use the thing several times, standing in front of it, to even have much of a chance of getting it, but still not something that you'd want to mess with.
As a side note: adding metal into the box in any way changes the fundamental frequency of the box, and could overload the magnetron that makes the microwaves. In other words, slowly break it.
A microwave works really well for bending smaller pieces of wood, like the recurves for a horn bow, if you stick the wood you want to bend in a cup full of water, and stick it in the microwave for a while, watching it close, and let it boil for a bit. Works great. If you could find a microwave big enough to do a shortbow, that'd be cool. Modifying one, not so much.
Waterlogged (year away from a physics degree)
