I was looking at these two, and they seem like they move in step. Occasionally though, one starts to move faster than the other and give better returns. What are the factors that drive the acceleration of one versus another? And, at least from my casual inspection, it seems like the one that's winning vs. losing can even alternate. I'm not an energy trader; I just started browsing casually at energy products and couldn't quite come up with an explanation as to why coal-based stocks get different performance from oil-based stocks, at least from a demand perspective. Supply perspective, I can maybe understand -- sometimes oil can be more difficult to extract, but I guess I'm looking for "the factor" that explains why one might do better than another.