So that's going to be the product of X and Y, and then this thing right over here, the expected value of Y, that can be approximated by the sample mean of Y, and the expected value of X can be approximated by the sample mean of X. So what can the covariance of two random variables be approximated by? What can they be approximated by? Well, this right here is the mean of their product from your sample, minus the mean of your sample Ys times the mean of your sample Xs. And this should start looking familiar. This should look a little bit familiar, because what is this? This was the numerator.