reCaptcha, how does it work?

Scans of old book pages are read by two different types of character recognition software. When a same word is read differently by these two software programmes, it is considered suspicious and is added to the reCaptcha database.
When you are asked to prove that you are human and not a machine using a reCaptcha, there are always two words, one is more deformed than the other. One of the two has already been identified as valid by OCR software (and this is the one used to verify that you are human) and the other one has not (this will be the word you help to identify). When a certain number of internet users identify a suspicious word in the same way it is then validated. It is then included into the database of validated reCaptcha words and into the database used by OCR software to recognise the characters in digitalised books. Today, the reCaptcha character recognition software has achieved a level of error similar to that of a human being.