I figured this was just a matter of time until someone implemented this. I wish there was some way to do this based on referrer or browsing habits or something so it was automatic, but then I guess the robots would mimic human behavior anyway…

Captchas were never intended to be purely visual — however, most initial implementations used fuzzy images, and in attempting to lock out automated agents also inadvertently locked out people unable to see the image. As an alternative to these, this past spring Google Services that require verification began to provide an audio alternative — people have the option of listening to a sequence of spoken digits that they then type into a form field to verify to the web application that there is indeed a live human at the other end.

Full Explanation

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