Just curious about how these things work. I notice that often VXX, XIV, and the others in the space sometimes move by different percentage amounts (and in different directions) than the Vix futures. For example, early today the Vix futures front and second month were down by about 1%, yet VXX was up by close to 1% and XIV was down by about the same amount. The VIX itself was down, and the ES futures were up. I would expect VXX to be down and XIV to be up. If these ETPs track the performance of the Vix futures, how can they be out of line with the those futures so dramatically? Also interesting is that the entire space was uniformly out of line (VXX, TVIX, UVXY, SVXY, XIV, TVIZ, and ZIV). Thus, across ETP companies, the aberration from the stated tracking goals has presented itself, often persistently. (And not just today either, but ubiquitously in recent days and weeks.) Any thoughts on how and why this happens?