You know this, but we'll just refresh your memory: The word gratuitous, as defined by Merriam-Webster, means "not called for by the circumstances; unwarranted."

A movie usually has a lot of ground to cover in 90 or more minutes; it must introduce characters, set up conflict, see that conflict through to its climax, and tie up loose ends if possible before the closing credits roll. 

So why then do we get anything gratuitous in film? Do we really have room for unwarranted digressions? Hollywood seems to give a resounding yes. If your movie is boring, confusing, implausible, or simply not sexy enough, Tinseltown's evident solution is to throw in some gratuitous girl-on-girl action and the head-bobbing audience will perk up a bit. 

This weekend, the movie Butter will open; in it we will see (in addition to amazing feats of butter-sculpting) Ashley Greene and Olivia Wilde share some steamy sapphic moments. While not strictly necessary, we're pretty sure this will make the wacky comedy a better film. Hell, it worked for these other 20 flicks. Enjoy this look back at the 20 most gratuitous girl-on-girl scenes in movies.