Here it is:
A function, f(x), has an inverse function 
I know what you're thinking:
"Oh, yeah! Thanks a heap, math geek lady. That's very helpful!"
Come on! You know I'm going to tell you what onetoone is! Have I let you down yet?
OK, onetoone...
There's an easy way to look at it, then there's a more technical way. (The technical way will really get us off track, so I'm leaving it out for now.)
Here's the easy way:
The Horizontal Line Test: If you can draw a horizontal line so that it hits the graph in more than one spot, then it is NOT onetoone. 
Check it out:
Is  onetoone? 
Yep!
