mystery
Explanation
This comic depicts a Halloween "mystery box" game -- a classic party activity where blindfolded participants reach into containers and are told they're touching something gross (like "peeled grapes are eyeballs" or "spaghetti is brains").
A person dressed as a witch dramatically announces, "And this box is filled with... human heaaaaarts!" while blindfolded children in costumes (one appears to be wearing a crown, another a superhero cape) reach into buckets that appear to be dripping with something.
The caption below reads: "The Halloween Mystery Box game got way more fun after cell-based tissue culturing was perfected."
The joke is that in a future where lab-grown tissue is readily available, the classic Halloween gag no longer needs to rely on substitutes -- the boxes can now contain actual human hearts (grown in a lab, not harvested from people). The humor is darkly absurd: biotechnology has advanced to the point where it's being used not for medical breakthroughs but to make children's Halloween games more authentic. It's a characteristic SMBC move of taking a technological advancement to its most trivial and disturbing possible application.