Which innovative design feature was deliberately incorporated into the final version of Thor's hammer Mjölnir during its mythological product development?

Throughout Norse mythology, Thor's hammer Mjölnir underwent several design iterations before becoming the legendary weapon we know today. This poll tests your knowledge about how this iconic mythological product evolved through ancient product development cycles. The story of Mjölnir represents one of mythology's greatest examples of divine product innovation, combining power, portability, and user-specific design features that modern product developers still reference as inspiration.

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The short handle, a 'design flaw' caused by Loki's interference with the dwarven smiths during forging, which made it usable only by Thor
The hammer's ability to shrink in size, allowing Thor to conceal it as jewelry when not in battle, added in the third design iteration
The self-returning feature, which was added only after the first prototype was lost in battle against the frost giants
The lightning enhancement, which was incorporated after market research showed users wanted weather control capabilities