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The story there is that he had written this in high school, combining the style of MAD magazine with the textbook “system of weights and measures”, as one of his submissions to the Westinghouse Science Talent Search (1956). A few months later when he was in college he sent it to MAD magazine with (basically) “you guys may like this”, and to his surprise they treated it as a submission and decided to publish it (with illustrations by Wallace Wood); it came out in June 1957. Later when writing his CV Knuth decided to start counting from this as his first publication.