Building a docx by Hand in a Text Editor
"A docx is just a zip, right? So if I zip up some XML and change the extension, I get a docx?"
I actually tried it, and it plainly failed. A docx is not merely a zip — the contents of the zip must follow a fixed package structure called OOXML (Office Open XML). Since I was at it, I decided to find out hands-on exactly how far you have to go to produce a genuine docx.