The Pearson product-moment correlation, r, is commonly applied in educational research. Almost all researchers realize that r ranges between -1.00 and +1.00, and that negative coefficients indicate that the bivariate relationship is inverse. Researchers also recognize that the Pearson r only evaluates linear relationship, and is not sensitive to curvilinear relationship. However, few researchers, if pressed, could explain exactly what makes r negative or positive, from a mathematical point of view, even though most researchers know what such results mean. The present paper explores the factors that affect r, including those that impact its sign. The reasons for preferring r over the covariance are explored. Small data sets and graphs are employed to make the discussion concrete. Four figures, four tables. (Contains 8 references.) (Author)