This is too cool, and probably far too easy, meaning that any idiot or 3 year old can do this which is kind of disturbing. Also, note the gasoline can he has sitting next to the microwave which he apparently noticed later during post production and thought prudent to warn you about with a little text “bad idea” sign during the video.
Here’s an explanation as to why what you just saw happens, and why you’ll get roughly the same effect if you cut a grape in half and stick in the microwave: Why do grapes spark in the microwave?