reCAPTCHA made every bot-check also digitize a word no computer could read

By the mid-2000s, CAPTCHA — distorted text that separated humans from bots at login and signup screens — was solving security but wasting enormous amounts of human attention: Luis von Ahn, the Carnegie Mellon computer scientist who had helped popularize CAPTCHA, calculated that roughly 200 million CAPTCHAs were being solved every day worldwide, at about ten seconds each, adding up to over 500,000 human-hours daily spent typing squiggly letters that vanished the instant they were verified.

Von Ahn's earlier project, the ESP Game, had already shown that gameplay could extract useful labels from players as a side effect of entertainment. CAPTCHA presented the same opportunity in reverse: it was labor people were already performing, at scale, for free, but the output — a solved squiggle — was thrown away the moment it verified a login.

Split the CAPTCHA into two words: one known to grade the human, one unknown from a scan, and cross-checked across users to transcribe it.

Users already accepted typing distorted text as the price of using a site, so adding a second word cost them almost nothing.

The unknown word's transcription was checked across many users, turning unreliable guesses into statistically confident results.

The marginal cost of the second word was near zero while the value of digitizing archives was enormous, making the trade worthwhile.

Attach your task to an existing high-volume activity as a second, ungraded item; the friction users already tolerate subsidizes it for free.

Digitized 13M+ NYT articles back to 1851 and ~2M books a year for Google Books, solved by over a billion people before Google retired it. reCAPTCHA became the standard proof-of-humanity widget across much of the web for close to a decade and established human-computation-as-byproduct as a recognized design pattern, directly informing von Ahn's next venture, Duolingo, which used the same idea to have language learners translate real sentences as they practiced.