critical
Explanation
The Joke
The comic depicts a grandmother at Christmas giving her grandchildren a gift. She says she never needed to buy frivolous things for herself and has instead spent her money on presents: "I've always wanted people to know how much was spent on it for them." She presents the children with a gift that includes Uranium-235 (U-235) and what appears to be a sizeable amount of it -- 80 units (likely kilograms). The children exclaim about how generous and selfless Grandma is, but the final panels reveal the punchline: the grandmother has given them enough fissile material to reach critical mass. The comic ends with the words "Christmas and Critical" -- a play on "Christmas and..." holiday specials, where "critical" refers to nuclear critical mass.
The joke builds on the trope of the selfless grandmother who saves up all year to buy extravagant gifts for her grandchildren. But the gift she has chosen -- enriched uranium -- is not a luxury item but weapons-grade nuclear material. The children's innocent gratitude ("Granny, you're so selfless!") is hilariously misplaced given that she has essentially given them the components for a nuclear weapon or meltdown.
The Humor
The comedy operates through escalating absurdity disguised as a heartwarming holiday scene. Every element of the grandmotherly gift-giving trope is present -- the self-sacrifice, the tearful gratitude, the emphasis on the cost -- but the gift itself is catastrophically dangerous. The punchline "Christmas and Critical" works as both a parody of holiday special titles and a literal description of what happens when you accumulate enough U-235 in one place. The bonus panel note about this being a companion piece to the book "Open Borders" adds another layer, suggesting this is bonus content related to a larger project about policy, making the nuclear Christmas even more incongruous.
References
Critical mass refers to the minimum amount of fissile material needed to sustain a nuclear chain reaction. For U-235, the bare critical mass is approximately 52 kg, so 80 kg would indeed be supercritical. The comic references the book "Open Borders: The Science and Ethics of Immigration" by Bryan Caplan, illustrated by SMBC creator Zach Weinersmith, which was published in 2019.