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Explanation
This comic is a single-panel gag with a morbid and darkly funny premise.
A family visits a grandfather's grave and notices that the circle of dead plants around the gravesite keeps expanding. The only thing growing is a "weird black mold." A child innocently asks, "Was Grandpa... the Devil?"
Below the image, a caption reads: "Death Pro Tip: Shortly before the end, swallow a balloon full of plutonium dioxide."
The joke operates on two levels. On the surface, the expanding circle of death around the grave is creepy and supernatural-seeming, leading the child to the logical (for a child) conclusion that Grandpa must have been the Devil. But the "Death Pro Tip" below reveals the mundane (and horrifying) explanation: the grandfather deliberately swallowed radioactive material before dying, so his buried body is irradiating the surrounding soil, killing plants and growing strange mold. It's a posthumous prank taken to an absurdly extreme level.
The humor is very dark -- it's essentially a guide to haunting your family from beyond the grave using nuclear physics instead of supernatural powers. The "Pro Tip" format parodies lifestyle advice columns, treating "how to make your grave site terrifyingly radioactive" as a helpful life hack. It's classic Weiner in its combination of scientific knowledge and pitch-black comedy.