George Uhlenbeck and Samuel Goudsmit, graduate students at Leiden under the supervision of Paul Ehrenfest, propose that each electron possesses, in addition to its charge, an intrinsic angular momentum — "spin" — of magnitude ℏ/2, along with a magnetic moment of one Bohr magneton. The idea resolves at a stroke several accumulated difficulties of atomic theory: the structure of spectral lines into doublets and multiplets, and the anomalous Zeeman effect. It is, in essence, the concrete physical interpretation of the mysterious fourth quantum number Pauli had introduced months earlier without knowing what it represented. When Uhlenbeck and Goudsmit present the idea to Ehrenfest, he responds that it might be nonsense or something very important, and asks them to write a brief note; Hendrik Lorentz, consulted afterward, points out that an electron spinning with those dimensions would have a surface velocity exceeding that of light — a serious classical objection — leading the two young men to ask to withdraw the paper. Ehrenfest, however, had already sent it, and reassures them with the famous line: "you are both young enough to be able to afford a stupidity." The note, published in Naturwissenschaften in November 1925, initially triggers rejection from Pauli and other physicists over a factor-of-2 quantitative discrepancy in the fine structure of hydrogen — a discrepancy Llewellyn Thomas would resolve months later. Electron spin is today revealed as a fundamental quantum property, shared by neutrons, protons, and most elementary particles, and proves indispensable for understanding the Pauli exclusion principle, the periodic table, the magnetism of matter, and, decades later, all spin-based quantum information physics.