Vertical Line Test

To see if a graph is a function,
do the vertical line test.

From the left side of the graph,
think of a vertical line.
Move the vertical line to the right side.
And see if the vertical line and the graph intersect at [one point].

As you can see,
the line and the graph intersect at one point.

If the line and the graph intersect at one point,
the graph passes the test.
So this graph passes the vertical line test.

The vertical line means [one x].
And one intersecting point means [one y].
So the vertical line test is testing
if the graph shows [one x, one y]:
a function.

So this graph is a function.

Do the vertical line test.

If the vertical line is on the origin,
the line and the graph intersect at one point.

So, at this point,
the graph passes the test.

But if the vertical line is moved to the right,
the line and the graph intersect at two points:
not at one point.

If the line and the graph intersect at more than one point,
the graph fails the test.
So this graph fails the vertical line test.

So this graph is not a function.

Do the vertical line test.

Move the vertical line from the left to the right.
And see if the vertical line and the graph intersect at [one point].

The line and the graph intersect at one point.
So this graph passes the vertical line test.

So this graph is a function.