ReCaptcha ensures Web site users are human by requiring them to type in distorted and obfuscated words. The computer knows what one of the words is; the other is an unknown word from a book that's being digitized. When three people type in the same result for the unknown word, ReCaptcha concludes the human digitization is accurate. The free Carnegie Mellon University service can be used to prevent computers from registering for e-mail accounts, entering spam comments on blogs, or reading e-mail addresses on Web sites.
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