Which innovative packaging design technique revolutionized children's literature in the 2000s by incorporating sensory elements that significantly boosted sales?

The art of book packaging has evolved dramatically over the centuries, from simple leather-bound covers to today's interactive designs. Publishers employ sophisticated packaging strategies to attract readers and differentiate titles in an increasingly competitive market. This trivia question explores a groundbreaking packaging innovation that transformed how readers interact with certain literary works.

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Thermochromic ink covers that change color when touched
Augmented reality markers embedded in illustrations
Touch-and-feel elements with varied textures integrated into storylines
Glow-in-the-dark binding that charges in daylight