Perhaps the simplest family of functions to become acquainted with is the linear functions. Linear functions include the constant functions (same output for every input), the functions that increase at a positive rate, and the functions that decrease at a negative rate. Also, it is this constancy that is the best way to recognize a situation in which linear functions will provide a good model. If we think of constant functions as increasing or decreasing at a constant rate of 0 (i.e., they don't increase or decrease at all).
An airplane flys at a constant altitude.
An airplane flys at a constant altitude.