So, I haven’t done too much reading on the copyright Supreme Court case, but there is debate about what is copyrightable. Think about something like the Happy Birthday song - sure, that one guy created it, but it had been used so many times and in so many contexts that the Copyright Office ruled that it was in the public domain now - no way to put the genie back in the bottle. A similar concept may apply for the Fortnite dances. If more and more people are “flossing” in a way that is divorced from the original creator and original dance, then that may now be in the public domain, which would prevent backpack kid from copyrighting it. Honestly, it seems stupid, but as a law student, these are all really interesting legal questions being asked.