What about a parabola lying on its side?
(I'll teach you about these later.)
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?
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.