With three or more points, it will normally be impossible to fit it exactly.
Residuals sum to zero.
Recall that if the correlation between two variables is zero, then the covariance between them is also zero. R 2 ranges between 0 and 1. It turns out to be important to estimate the variance of the error terms. Correlation vs. Causation After doing all of this math, it is very easy to overestimate how far we have actually gotten.