Which famous invention was originally created for a completely different purpose until customer feedback revealed its true potential?

Throughout history, many groundbreaking inventions and discoveries were shaped by unexpected customer feedback or user insights. Sometimes the original purpose of a product evolved dramatically based on how people actually used it. This poll explores a famous case where consumer behavior completely transformed an inventor's vision, leading to one of the world's most recognized products today.

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Post-it Notes, which were originally developed as a strong adhesive but failed and was later repurposed when a user wanted bookmarks that wouldn't fall out
Coca-Cola, which was originally developed as a headache medicine until consumers reported enjoying it primarily as a refreshing beverage
The microwave oven, which was designed as a military radar device until an engineer noticed it had melted a chocolate bar in his pocket
WD-40, which was created to protect the Atlas missile from rust and corrosion but became popular after employees secretly took it home for household uses