I posted about Captchas–those squiggly barely-decipherable words used to filter robots from accessing protected websites–two years ago. Their story is fascinating, not well-known, and worth mentioning again. Even the name is cool: Captcha is an acronym for “completely automated public Turing test to tell computers and humans apart.” As the NY Times reported Monday Captchas serve a purpose in addition to separating humans from machines: they are bits of text from old books that require correction after the books have been scanned. Invented by researchers at Carnegie Mellon University they are used by the Google Book Project, the New York Times, and others transforming old and archival materials into accurate digital copy. It’s brilliantly simple: allow thousands of computer users to interpret unclear text snippets, collate the results, deduce the correct meaning. The Carnegie Mellon researchers estimate that “humans around the world decode at least 200 million Captchas per day, at 10 seconds per Captcha. This works out to about 500,000 hours per day . . .” This explains why some Captchas are impossible to decode. They are flyspecked text from musty centuries-old books.