First off, take a girls virginity. When she starts to bleed, get some blood on your fingers and smear it around your mouth from cheek to cheek. When she starts to cry or look disgusted, yell out "Why so serious?!"
Jenny started to bleed from the vagina, because it was her first time having sex. John then began to smear her blood all around his mouth from cheek to cheek, pretending to be the joker. As Jenny started to cry, John then proceeds to yell out "Why so serious?!"