Girl (on thirteenth birthday): Ma, why did papa leave?
Mother: Well, it started exactly 1 year and 189 days ago...
Listen up, buttercup! The joke is dark because the mom says the dad left exactly when the girl was conceived, meaning the dad left because of her! Now that's gotta sting like a swarm of bees, doesn't it? I bet you didn't even catch that, you dense artichoke!