10 Movie Secrets They'll Never Tell You

It may seem like a relatively trivial mystery compared to most of the entries on this list, but all the same, the question has been burning in the minds of fans for almost 35 years now - what the hell is "lithium mode" in Back to the Future Part II?

After Marty (Michael J. Fox), Doc (Christopher Lloyd), and Jennifer (Elisabeth Shue) travel to the future of 2015, Jennifer spies upon a less-than-happy middle-aged Marty in the McFly household.

The sequence provides glimpses of various futuristic technologies, the most bizarre and unexplained of which is "lithium mode."

After entering his home, Marty presses a button only for a voice to suddenly say, "lithium mode on," before Marty complains about his kids turning it off.

Now, it's not exactly a secret that lithium-based drugs are used to treat depression and bipolar disorder, but why exactly would Marty's house have a mode for that?

Or is it simply a power setting for the house, given that lithium is also a common component of batteries?

Don't expect either director Robert Zemeckis or writer Bob Gale to ever elaborate on the mystery, though, given that on the movie's DVD commentary they flat-out confirm that they never actually decided for themselves what lithium mode is, but simply thought that it sounded futuristic. Cheeky.