Graphical display of data and models
Before applying statistical methods to any data set, we should try to display the data graphically. For a situation with response Y and two explanatory variables, X and Z, a rotating 3-dimensional scatterplot can display the data.
This kind of 3-dimensional diagram can also effectively illustrate linear models and their parameters.
(Similar graphical displays are not possible when there are 3 or more explanatory variables, but the concepts are the same.)