"Doctor Who" was first shown in 1963, with William Hartnell as the Doctor. When Hartnell left the show in 1966, the writers came up with the idea that the Doctor would "regenerate" (at the time called "renewal") and transform into the Second Doctor, who was played by Patrick Troughton. In 1970, when Jon Pertwee took over as the Third Doctor, the series switched from black-and-white to colour. Before 1978, the BBC would sometimes throw away episodes they did not need. As a result, 97 of the 851 episodes are missing (however some of these have since been animated, since audio recordings of all missing episodes exist).