They reorder so that the thread with the latest post is at the top, then it stays in that relative position to the threads next to it until it gets bumped up again. So if you were to go back to the very first thread ever posted, and add to it, you'd bump it up to the top, and it would stay there.

Wait a second! You've read all of it?! *does a happy dance* Someone with less life than I've got!!!! Yes!

*laughs* It's all good, packman. I'm just amazed that anyone would do that.