As they are waving to everyone, Biden leans towards Trump saying,
“Did you know, that with one little wave of my hand, I can make every person in this crowd go wild with joy? Also, this joy will not be merely a momentary display, like that of your followers, but will go deep into their hearts, and for the rest of their lives, whenever they speak of this day, they will rejoice!”
Trump replies, “I seriously doubt that, with one wave of your hand? Come on, show me!”
So Biden slapped him.