Is it a cat, or is it a dog?
Not a stupid question, it appears, just one that computers have difficulty solving. In yet another example of why spammers do genuinely deserve huge jail sentences (a small inconvenience becomes a major crime when you do it to a billion people every day), the average captcha has now had to become so complicated that humans are starting to struggle, reports the NY Times.
One alternative is Asirra, which shows an array of photos from a lost pets website (and hence has a ready supply of new pictures). Just pick which are cats.
Not that the original creators of the captcha are giving up - Carnegie Mellon has now created the Recaptcha, which uses blurred script from old books - two recaptchas, one already cracked, the second unknown. More difficulty for the computer, and it helps turn old books into computer text when OCR can't handle it.