What if we tried this?
 

input: 101 DALMATIONS  ->  rule: spit out the first letter of the movie  ->  output: ?
 

Hmm... 101 Dalmations starts with a number, not a letter...  So, we can't even put it in the box!

(Think about it...  Where would they have this movie at the video store?  Before the A's!)

Here are some official math terms:

The stuff that goes IN the box (the INPUT)
is called the DOMAIN.

The stuff that spits OUT of the box (the OUTPUT)
is called the RANGE.

 

input: ALIEN, JAWS, STAR WARS = domain  ->  rule: spit out the first letter of the movie  ->  output: A, S, J = range

Domain guys go in...  Range guys spit out.

(If you forget the order, it's alphabetical D 01-functions-04a.gifR.)

So, for this box...

domain = all movies that start with a letter
(101 Dalmations is not in the domain.)

range = the alphabet

And, yes, there are movies for each letter.  Here are some for the weird letters:

Q  =  Queen Elizabeth (art film)

 X  =  Xanadu (bad disco movie)

   Z  =  Zulu Warriors (pure action!)