Which of these famous product discoveries was originally an unintended side effect of a failed experiment?

The history of inventions and discoveries is filled with products that revolutionized their industries and became iconic symbols of innovation. Some were happy accidents, while others were the result of decades of research. This trivia question tests your knowledge of famous products that changed the world and the fascinating stories behind their creation. Can you identify which famous innovation came about in this unexpected way?

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Post-it Notes (The adhesive was originally deemed too weak for its intended purpose)
WD-40 (The formula was perfected on the 40th try specifically to prevent rust)
Coca-Cola (It was deliberately formulated as a medicinal syrup to cure headaches)
Velcro (It was purposely designed after studying how burrs stick to dog fur)