Shodor Education Foundation
Shodor Interactivate: Lesson: Functions and the Vertical Line Test
This lesson plan outlines an activity that allows students to explore the vertical line test for functions.
Texas Instruments
Texas Instruments: Relations and Functions
A function is a relation that assigns exactly one value of the dependent variable to each value of the independent variable. When a function is graphed, a vertical line can pass through the graph and touch only one point.