It's already been said, but with reCaptcha you are shown 2 different images and you only have to get the "known" one "correct". The other image is "unknown" and any answer is acceptable. It's usually pretty obvious which image is known and which is unknown.
If your answer for the "known" image is "correct", then your answer for the "unknown" image is trusted and saved. Once that unknown image gets a lot of the same answers, it becomes known, with that as the answer.
The problem with reCaptcha is that everything is illegible shit now, and the spammers have beaten it anyway. A lot of the "known" words have wrong answers because spammers used automate tools to guess at them. This is why you can type the known word in correctly and have the thing tell you "LOL NOPE" - spammers have poisoned the database by using automate tools that guessed the same answer for a given unknown image hundreds or thousands of times, while getting the right answer for the known image up to 60% of the time.
Just use the audio option for blind people. I've never had a problem with it (though it looks like others have).