Do current methods of CAPTCHA exactly work

A CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is a program that can tell whether its user is a human or a computer.

The process involves a computer asking a user to complete a simple test which generated by computer. Because other computers are unable to solve the CAPTCHA, any user entering a correct solution is presumed to be human. Sometimes it is described as a reverse Turing test, because it is done by a machine and targeted to a human. reCAPTCHA does exactly that by channeling the effort spent solving CAPTCHAs online into "reading" books.

reCaptcha is hosted by Google, and one of the more interesting things about it is that it is used to digitize text of old newspapers and books. That’s why there are two “sections” of a reCaptcha instead of the single series of characters for CAPTCHA - one is known text, the other is not. If you get the known one correct, it assumes you got the second one. Then the next time it offers up that same “unknown” text, it is considered possibly known.

A few more times with the same result for the “unknown” text, and it becomes “known” and the text it originated from can be correctly digitized. Clever, eh?

Also, because of frequent updates, I would expect reCAPTCHA to be slightly better at preventing bots from solving them.