Ok, so the only rule here is... post one song as a response to the previous song and just keep it going! It ends up turning into an interesting musical telephone game that sometimes even creates a story of its own, haha. Your response to the previous song can be inspired by anything. Maybe it sounds similar in some way, or the lyrics reminded you of something else, or it somehow ties into a memory of yours that connects to something else, reminds you of a character so you post another song that reminds you of that character, etc. Anything goes, really.