First, put your pencil or ruler up against the screen.
Move it down the graph and if there are two points in one x point, it is not a function. For example, look at 0, 3 and 0, -2.5. They share the same x point.
In the graph of a circle, we can easily draw a straight vertical line passing through its center. This will hit the top and the bottom of the shape. Since this line hits two points, a circle's graph is not a function. We can also observe the equation of a circle.
