Now, what is this over here? I just said you could rewrite this very easily. This bottom part right here, you could write as the mean of X times X, that's the same thing as X squared, minus the mean of X times the mean of X. That's what the mean of X squared is. What's this? You could view this as the covariance of X with X. We've actually already seen this, and I've actually shown you many, many videos ago when we first learned about it what this is.