CAPTCHA is a test that many readers have seen but likely do not know what it is. CAPTCHA stands for "Completely Automated Public Turing test to tell Computers and Humans Apart." CAPTCHAs consist of small images of letters or words that one must type in to convince a system that you are in fact a human and not a spambot. The letters are generally somewhat distorted or have stray additional lines added. Such excercises take advantage of the fact that humans are able to do difficult symbolic recognition processing that computers so far cannot.
One more recent innovation is reCAPTCHA. reCAPTCHA helps to solve an interesting problem: There are long-term efforts to enage in mass digitization of old books and records. However, there is a problem: simply saving images of the pages would take up much space and would make the pages unsearchable. Thus, books are being scanned and computer programs are being used to figure out what the words are. Some words however, if they are scratched, poorly written, water-damaged or subject to other problems can make the computers unable to recognize the original wording. This is the same issue that allows CAPTCHA to work. However, it is impractical to have humans comb through these many words to identify them all. Now, enter reCAPTCHA.
reCAPTCHA is just like CAPTCHA but the source words to be deciphered are words from old books which computers are having trouble recognizing. The same words needing to be recaptured are presented to multiple different users. If the users agree then the digitizers can be pretty sure that the humans successfuly recognized the correct word and can then digitize that word and use it as an additional word as a challenge word.
More specifically, each reCAPTCHA challenege consists of two words. One of which has a known answer and one which does not. The individual challenged does not know which word is in which category and thus must answer both.
This procedure is a brilliant way of harvesting otherwise lost processing power.
Now, this is all well and good, but what am I expected to do when the reCAPTCHA challenge is:
Large gaps between consecutive prime numbers
16 hours ago