Spurious correlation is the correlation built into ratios, products, or other functions of two sets of random variables when the two sets have some elements in common. In hydraulic and hydrologic literature, many instances may be found of spurious correlations that are mistakenly interpreted as establishing relationships between single elements when such relationships may not exist or may not be as closely defined as they appear to be. Examples are given from technical literature. Tables are presented giving means, standard deviations, and correlation coefficients for ratios and products of variables, for both general and specific cases. Caution is advised in the use of functions with common elements.