Watching BTTF3 you can’t help but notice that Doc Brown’s kids are a little creepy looking and weird. But one thing that stands out in this scene is the hand motions that the youngest makes. Watch Vern as Doc is talking to Maggie. To give him the benifit of the doubt, he could be saying “mommy, after this scene I need to pee”, or he could be saying “Hey Michael, wanna ride on my disco stick?”. You decide.