Vertical line test

Property If a vertical line intersects the graph at more than one point, then the graph is not a function.

Examples A parabola like $y = x^2$ passes the test. Any vertical line drawn will only intersect the curve at a single point, so it is a function. A straight line like $y = x$ also passes the test. Any vertical line will only cross it once, confirming it is a function. The graph of a circle, like $x^2 + y^2 = 1$, fails the test. A vertical line drawn through its center would hit both the top and bottom of the circle.

Explanation Imagine scanning a vertical laser beam across your graph from left to right. If that beam ever hits your graphed line in more than one place at the same time, it fails the test! A true function gives only one output (y) for each input (x), so the laser should only ever touch one point at a time.