Comment Re:Good idea, but how? (Score 1) 138
Simple.
They present two words - one is computer generated and is, in fact, the real CAPTCHA test. The other is a failed to OCR word from a book. People fill both words, because they don't know, which is which. They show the same failed OCR word to a hundred people and get a stable result by majority of people, even if somebody tries to abuse the system and write some bad words instead.