the vertical line test is a graphical way to determine whether or not the graph is a function of x
if you can draw a vertical line on a graph and cross the graph at least 2 times, the graph is not a function
a function will have 1 y value for every x value, therefore, if we can draw a vertical line (x=c, a constant) that intersects the curve at more than 1 point, then that point where it intersects at more than 1 point is where 1 value of x coresponds to more than 1 value of y and is therefore not a function
