Hi Bob.

If you have a graph of a relationship between two variables x, (horizontal) and y (vertical), then in order for the relationship to describe "y as a function of x", there should be no place on the graph where you can draw a vertical line and have it cross the curve more than once.

For example, a circle is not a function, because you can draw a vertical line through it and have it pass through the curve twice (top and bottom). A parabola opening upwards is a function because any vertical line you choose to draw cannot cross the parabola more than once.

Hope this helps,
Stephen La Rocque.