two assets moving together tells you what they did. nothing about what they owe each other. correlation is a mood. it evaporates the moment one leg catches a headline and the other doesn't. cointegration is a debt. the spread can wander, but the further it stretches the more it owes. that obligation is the entire trade. we pass on correlated pairs every day. hurst above 0.5, cointegration false, z score screaming. looks like free money. it is a coin flip wearing a lab coat. the pairs worth your size are the ones the math says must come home, even when the story says otherwise. most of the time the story wins for a while. the math just has to win eventually, and size accordingly.