Luis von Ahn invented CAPTCHA - the system that uses a picture of a word to determine whether you are a human or a computer. But when von Ahn realised just how much time was being wasted by filling in CAPTCHAs, he wondered whether that man-power could be used for something good. Now the very same process is digitising millions of books, that are available for free, as von Ahn told Chris Smith. Like this podcast? Please help us by supporting the Naked Scientists