We haven't discussed this yet, but in Ruby we don't typically care what class on object is, we just care that it responds to the methods that we need to call on it. For example, if we passed in an `Article` object, but it still responded to `title` and `author`, we would still want it to be a part of the library.

This is what's referred to as "duck typing" - if it walks like a duck and talks like a duck, we'll treat it as one.