Which retail innovation, introduced in the early 20th century, first allowed customers to select their own merchandise without clerk assistance and revolutionized shopping behavior?

Throughout history, retail innovations have transformed how people shop and how merchants sell. From ancient marketplaces to modern digital storefronts, the evolution of retail reflects broader societal and technological changes. This trivia question focuses on a pivotal innovation in retail history that fundamentally changed consumer behavior and set the stage for modern shopping experiences.

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Self-service grocery stores (Piggly Wiggly, 1916)
Department stores with multiple floors (Marshall Field's, 1902)
Mail-order catalogs with photographs (Sears Roebuck, 1894)
Window shopping displays with mannequins (Macy's, 1923)