metal
Explanation
In this single-panel comic, a doctor cheerfully tells a patient: "Good news! Your body is over 90% metal!" The patient, sitting on an examination table in a hospital gown, looks unamused and skeptical.
The caption below reads: "It was a mistake to allow astronomers into medicine."
The joke hinges on the different definitions of "metal" used in astronomy versus everyday life. In astronomy, the term "metal" (or "metallicity") refers to any element heavier than hydrogen and helium. Since the human body is composed primarily of oxygen, carbon, nitrogen, and other elements that are all heavier than helium, an astronomer would technically classify the body as being overwhelmingly "metal." In everyday medicine and chemistry, however, "metal" refers to actual metallic elements like iron, zinc, and copper, which make up only a tiny fraction of the human body.
The humor comes from the absurdity of applying astronomical terminology to a medical context, where "your body is 90% metal" would be alarming rather than trivially true. The doctor's cheerful delivery of this "good news" adds to the comedy -- from an astronomer's perspective, this is indeed perfectly normal and unremarkable, while from a patient's perspective it sounds terrifying. It is a classic SMBC joke built on the gap between technical jargon in different scientific fields.