The concept of interaction arises in the context of continuous functions. Verbally, it is the action of one independent variable on the action of another, and vice versa, on a dependent variable. Quantitatively, it is the second partial derivative of a function with respect to two independent variables. Misconceptions have arisen from attempts to understand action and interaction in discrete terms. The algebraic expressions for the actions of diuron and phorate on the fresh weight of oats and their interaction were determined from the regression polynomial and plotted in three-dimensional graphs. Three other mutually incompatible methods of assessing interaction, namely, the Colby (with its modification, regression estimate), the two-parameter, and the topographical methods were evaluated.