Washington D.C.'s research organizations — the so-called "think tanks -- provide opinions and research on public-policy issues for lawmakers and the U.S. government’s Executive Branch.  But they also serve another function.  These "think tanks" commonly house officials from previous administrations who often return to government when their political party comes back in power. In this third segment of our "Think Tank" series, VOA's Jeffrey Young looks at those who go back and forth through what's often called the "revolving door."