From what I understand, Captchas are text that have been distorted by the application of filters, noise and other miscelaneous algorithms. Therefore, to find out whether the person's reading ability is that of a person, you compare what they answered to what the known answer is.
Now, reading up on ReCaptcha, it says that the words that are displayed are those that cannot be translated by OCR. In addition, recaptcha is being used to translate those images. How can it tell whether you are indeed right in your reading or are just making stuff up?
If it knew what it said, it wouldn't be used in recaptcha as translation material. If it doesn't know what the text says, then how does it validate your answer?
I'm guessing this is probably some probability based analysis with huge sample sizes before it flags anything as translated.
Does anyone know where the answer to this is?