Now I know why using reCAPTCHAS is a good idea. It's helping to digitize books that can't be simply OCR'd: "In a two-word ReCaptcha, one of the words you type is the computer generated puzzle that proves you're a human. The other word comes out of the [Internet] archive. It's assumed that if you can decipher the first word correctly, you can do the second word too. If 10 people decipher the word the same way, it is officially digitized."
Thank you to Judy Sims' blog comment from 2009 [linked above] - which was sparked by an explanation on Ticketmaster's page explaining the idea & then googling reCAPTCHA