So the way Tumblr works is that your account is basically a blog, with your home page on the site being populated with posts from the accounts that you follow. You can reblog posts onto your own account and comment on them to create individual conversation threads like this one. At one point, there was a bug in the edit post system that let you edit the entirety of a post when you reblogged it, including what other people had said previously, and even the original post. This would only affect your specific reblog of it, of course, but you could edit a post to say something completely different from the original and create a completely unrelated comment chain.

John green sends his regards