What about a parabola lying on its side?

(I'll teach you about these later.)

a graph of a standard parabola lying on its side a graph of a standard parabola lying on its side showing that a vertical line intersects its graph at two points

Ouch!  This guy hits in two spots!

So, Sideways Parabola Guy is not a function.

The only problem with this method is that you don't always have a picture to look at.

(There are other ways to tell that I'll show you later.)

Which of these are functions?

a graph of a line with some slopea graph of an "s"-like curve that passes the vertical line testa graph of an "s"-like curve that does not pass the vertical line test

The first two are...  The first guy is just a line.  He's officially called a linear function.

What's the only type of line that isn't a function? ________________

The second guy passes the vertical line test, so it's a function.

The last guy fails the vertical line test and is not a function.