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How do unsolved puzzles become anomalies in Thomas Kuhn's account of science?

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Marble bust of Aristotle with a beard and draped robe, showing detailed facial features and wavy hair, set against a dark background. — Philosophy of science Aristotle, marble portrait bust, Roman copy (2nd century bce) of a Greek original (c. 325 bce); in the Museo Nazionale Romano, Rome.

In Thomas Kuhn’s account of science, the transition from a puzzle to an anomaly is a shift in how the scientific community perceives persistent failure within a paradigm.1

During periods of "normal science," researchers work within a shared conceptual framework to solve well-defined problems, or "puzzles." When a scientist fails to solve a puzzle, the failure is typically attributed to the individual researcher’s lack of skill rather than a flaw in the paradigm itself.1

However, when these puzzles prove to be "recalcitrant"—meaning they repeatedly resist solution despite sustained effort—the scientific community begins to develop a sense that something is fundamentally amiss. At this stage, the unsolved puzzles are reclassified as "anomalies". These anomalies expose the inadequacies of the existing paradigm or contradict it entirely. If these difficulties accumulate, they can trigger a crisis, potentially leading to a scientific revolution where the old paradigm is replaced by a new one.12