belly-button
Explanation
The Joke
A little girl asks her father the classic child question: "Daddy, why do I have a belly button?" The father begins explaining that a long time ago, the belly button is where mommy was connected to her -- but then he gets the process exactly backwards. Instead of describing how the umbilical cord allowed the mother to nourish the baby inside her body until it was ready to be born, he describes the reverse: mommy pulling the child into her body and absorbing the child's nutrients until the child "ceased to be."
He catches himself mid-explanation with a confused "Wait, is that right?" The caption at the bottom completes the joke: "Sometimes, I forget which way the arrow of time points." The father has accidentally described pregnancy in reverse -- as if rewinding a video of gestation and birth, turning the life-giving process into something that sounds like horrifying absorption and consumption.
The Humor
The humor comes from the dad's earnest confusion about which direction biological processes run when viewed through time. Played forward, pregnancy is beautiful and life-affirming; played backward, it sounds like a horror movie where a woman absorbs a child through a cord and consumes them until they disappear. The joke is both a clever observation about the arrow of time (a real concept in physics) and a parody of the fumbling parent who cannot quite explain biology to their kid. The child's innocent question meets an answer that is technically accurate -- just in the wrong temporal direction.
References
The "arrow of time" is a concept from physics referring to the one-way direction of time from past to future, closely related to the second law of thermodynamics and the increase of entropy. The comic plays on this concept in a biological rather than physical context.