One square of dots, built to forgive. Follow a QR code from first glance to the stain it survives.
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This is a curated, step-through lesson, not a scanner. The square is a real version-1 QR code holding the word HELLO, drawn so you can watch its parts work; the smudge step simulates what a reader rebuilds, it does not run a real decode. Level H can restore roughly 30 percent of total codewords (8-bit units), not any arbitrary 30 percent of the square. Damage location, landmarks and capture quality matter; this illustration does not prove the shown stain is recoverable. Challenge answers are practice, not a measure of mastery.
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The lesson shows one small code. Real QR codes grow through 40 versions, add mask patterns and format bits, and carry far more than five letters. What stays true is the order of ideas: landmarks, rulers, data, spare letters.