When the MPAA gives a film a PG, PG-13, R, or NC-17 rating they like to say why. Usually it's pretty ordinary stuff: "language, sexual content" and the like.
...Sometimes, though, it's hilarious.
My initial source is this: thoughtcatalog.com/ted-pillow/2013/02/the-50-most-bizarrely-funny-movie-ratings/ Since then, I've added in many others I've noticed.
Suggestions are welcome!