Now, we’re used to Spotify. Of course, every song we could ever want to listen to is available as soon as it’s released. But, as integral as the service has become to many people’s daily lives, parts of it remain a mystery. Why the heck does all that folk-punk keep ending up on your Discover Weekly playlist? And how does it determine which tracks show up in the “Popular” area on artist pages? We have an idea, but we don’t know for sure.