While having sex you finger the girls ass and make sure to get her poo on your finger. When ready, you pull out (before during or after cumming as it does not matter.) and smear her poo from one cheek, across her lips to the other cheek. When she gets mad at you, simply ask "Why So Serious?".
I gave her the joker last night. After I smeared her poo from one cheek to the other via the lips she got all mad at me. So I asked her "Why so serious?"