Microwave Mystery

It bothers me greatly that I don't understand how microwave ovens work.

I mean, I get the gist of how most of our main appliances work.  You stick bread into the toaster, and you can see those little red things get hot, and that's how the toast gets burned to a crisp.  The blender has a little propeller thing in it, so when you turn it on it makes a really loud, annoying noise and it makes things mooshy.

The stove is easy enough to understand because you can actually see flames making stuff hot.  No problem.  But microwave ovens don't make any sense at all.  Nobody seems to understand them.  You stick stuff in there, and you can't see that anything is happening, but sure enough, your food quickly becomes heated.

Microwave ovens have been around for a long, long time, too.  According to Wikipedia, some dude named Percy figured out how to microwave stuff way back in the 1940s.  This is remarkable for a couple reasons... back in the 40s people didn't know a whole lot of stuff, considering Al Gore had not yet invented the Internet.  And even more remarkable is the fact that this guy's name was Percy.  Who the heck names a kid Percy?  Even in the 1940s, that couldn't have been cool.

When we were growing up, my mom would never allow us to have a microwave oven for fear that they'd melt our brains, or something.  So my only exposure to them back then was when we went to visit my grandparents.  Most kids were not as amazed by microwave ovens and their miraculous abilities to melt cheese over Doritos as I was.

Ever wonder what would happen if you were able to keep the door of the microwave open and turn it on?  I mean, would everything in its path start bubbling and melting all over the place?  I've never heard of anyone attempting it, but surely I'm not the only one who wonders such things.  I would certainly be tempted to open the door up and aim the microwave at our Swearing Neighbors as an experiment.

Oh, and another thing: why can't we put metal in there?  Just another microwave oddity, if you ask me.

Anyway, if anyone can explain to me in simple terms how these things work, I would be tremendously grateful.  I would assume that the real answer is something along the lines of "alien technology."