Everybody's doing the thumb thing.

You know how on Youtube and many other sites you can either Thumbs Up
+ or Thumbs Down - a news story or comment. Okay, well I want to do
this. I have a Model, Controller, Etc, for it, but I will use it for
all of this:

* Pictures
* Comments
* News Articles

So, how can I set the model to -maybe- belong to many things. For
example Rating could belong to News_Item. But then I have a Rating
that belongs to Comment. How should I set this up in my database and
in my RoR model?