exist
Explanation
The Joke
A child asks his father, "Daddy, do I... how can I be sure I exist?" The father, startled ("Oh my gosh, Billy!"), tells the child "That is way too big of a question for such a little kid!" He then reassures him: "You're as real as this house, real as Mom and Grammy!"
In the final panel, the scene pulls back to reveal the father is a demonic or monstrous figure with horns and clawed hands, sitting in darkness — implying that neither the father, the child, nor possibly the entire scene is "real" in any conventional sense.
The Humor
The comic sets up a wholesome parenting moment — a father comforting his child about an existential worry — then pulls the rug out by revealing that the father's reassurances are worthless because the entire situation is fundamentally questionable. The father says "you're as real as this house" and "as real as Mom and Grammy," but the visual reveal suggests none of those things may be real either.
The joke works on two levels: first, as a horror-style twist where the comforting parent turns out to be something sinister; and second, as a genuine philosophical joke — the father's argument from analogy ("you're as real as these other things") is logically circular if none of those reference points are real themselves.
Broader Context
SMBC frequently plays with existential and philosophical questions, often framing deep epistemological problems through mundane domestic scenes. This comic channels the Cartesian demon thought experiment — the idea that an evil entity could be fabricating your entire experience of reality — but delivers it through the format of a parent-child bedtime conversation.