"[Leaving Google] was an extremely difficult choice. I am as passionate about Google's mission now as I've ever been, and as proud of the work we're doing to achieve it," Silverstein said in a farewell email obtained by AllThingsD. "While a lot has changed at Google over the years, I think we've done a remarkable job of staying true to our core mission of making the world a better place by making information more accessible and useful. I am looking forward to pursuing that same mission, though in a slightly different way, at Khan." </p>
<p>Silverstein will work as a developer at Khan, which is also located in Mountain View, Calif.