Vertical line test

The vertical line test is a simple way to determine if a graph represents a function. Imagine drawing vertical lines (up and down) across the graph. If any vertical line touches the graph at more than one point, then the graph is not a function because a single input value (x-coordinate) corresponds to multiple outputs (y-values). Conversely, if each vertical line intersects the graph at only one point, the graph passes the test and is a function. This visual check helps identify whether a graph assigns exactly one output to each input.