Explain SMBC — the wiki for Saturday Morning Breakfast Cereal

The Joke

A student asks the teacher if they can tell them the sum of the angles of a triangle. The teacher answers "180 degrees." The student then asks a follow-up: "So we're just being told positively that this classroom exists in a pocket of negatively curved spacetime?" The teacher hesitantly confirms. The student presses further, asking if the teacher has been checked for "relativistic phenomena or just a principal's mandate." The teacher responds that the fault lies with "principal Einstein."

The comic plays on the mathematical fact that the angles of a triangle sum to exactly 180 degrees only in flat (Euclidean) geometry. In curved spacetime -- as described by Einstein's general relativity -- the angles of a triangle can sum to more or less than 180 degrees depending on whether the curvature is positive or negative. The student takes the teacher's definitive "180 degrees" answer and uses it as proof that their classroom must exist in perfectly flat spacetime, or else the answer would be different.

The Humor

The humor comes from a student weaponizing advanced physics knowledge against a basic geometry lesson. The teacher is giving the standard textbook answer, which is perfectly correct for a classroom setting, but the student pedantically points out that this answer implicitly makes assumptions about the curvature of spacetime. The "principal Einstein" pun at the end ties it together -- "principal" as in the school authority figure, and Einstein as the principal architect of general relativity. It is a classic SMBC move of having a character take a simple premise to its absurd logical extreme.

References

In non-Euclidean geometry, the sum of angles in a triangle differs from 180 degrees. On a positively curved surface (like a sphere), the sum exceeds 180 degrees; on a negatively curved surface (hyperbolic geometry), it is less than 180 degrees. Einstein's general theory of relativity describes gravity as the curvature of spacetime.