"reCAPTCHA improves the process of digitizing books by sending words that cannot be read by computers to the Web in the form of CAPTCHAs for humans to decipher. More specifically, each word that cannot be read correctly by OCR is placed on an image and used as a CAPTCHA. This is possible because most OCR programs alert you when a word cannot be read correctly."
corin_ hit the hammer on the head with that.
Sory, I should have tossed a link in when I made that comment.
There are always two words that you will see. One that is known, and the other is unknown.
Once enough users (lets use 10 for this example) type the same responce to the unknown word (and correctly identify the known word), the unknown word will move to a known status and be matched up with a new unknown word.
Thereby digitizing books! (ever wondered why one of the words is clearer than the other? Now you know!)
Are they actually still doing that? For the last year or so, recaptcha seems to only serve up nonsense words.
Only one of the words is actually known and used to verify humanness. The other is an unknown scan from a book, and after a certain number of identical entries (3?) is considered known.
That reliance on "certain number of identical entries" was used by 4Chan's gaming of the Time 100 poll in 2009. The so-called "penis attack": http://musicmachinery.com/2009/04/27/moot-wins-time-inc-lose...
Can you elaborate? What do you mean they are doing something useful? I'm not familiar with how recaptcha works but, I have used it on many sites before.
(Thanks!)