Let's do another one:

Think of a number -- make it positive...

Now, square it...  Then, take the square root of that.  What do you get?

The number you started with!

Check it out:

4  ->  ( )^2  ->  16  ->  square root( )  ->  4
 

So, these guys are inverse functions:

f( x ) = x^2

g( x ) = square root( x )