We don't know how much changed under the bonnet of Twitter's earthshaking UI overhaul today, but the trending topics algorithm is always going to be a big part of how the service operates. NPR has a piece up that explores just how topics and hashtags get automatically selected for the coveted trending column. In short, it's not simply a measure of how much a certain subject is being tweeted in general but how relevant and new the topic is. This explains why Tyson Chandler is currently trending and, say, Justin Bieber isn't, despite the formerly mop-haired Canadian singer being nearly constantly talked about at any given moment — speculation over Chandler's NBA future is both recent and rampant. We may or may not have noticed #PrayForJustinBieber trending a couple of weeks ago due to rumors of an asthma attack, however, which suggests that the algorithm has thresholds that can be broken by sudden spikes in activity. It's food for thought while you're deciding what to post with the shiny new interface.