By applying the vertical line test, one can assess whether different geometric shapes graphed on a coordinate plane are functions. Consider a vertical parabola that opens upward. Any vertical line drawn down the coordinate system will intersect this U-shaped graph at most one time. Because no vertical line hits it twice, every $x$-value maps to a single $y$-value, meaning the upward-opening parabola represents a function. Conversely, consider the graph of a circle. Drawing a vertical line through the inside of the circle will result in two intersection points (one on the top half and one on the bottom half). This indicates that a single input $x$-value corresponds to two separate $y$-values. Because it fails the vertical line test, the circle does not represent a function.