Have a reason, and not just, "Brookings is Great. Important. The Best." Definitely not, "I want to be great, important, the best one day, and Brookings seems like a good stepping stone." Perhaps you greatly admire a specific scholar's take on a specific policy, and particularly enjoyed a specific media appearance when said scholar really nailed their point home. Or you were at a social function and speaking with [name drop] and Sec/Sen/Rep/Director Bigwig, who told you, "Inspirational message." You were inspired, and you know Brookings is leading the way on [important issue]. Your mom/dad/formational character always told you [foundational value], and Brookings, because of its work on x, y, z, is the best place to defend that value. Make sure your reason is personal, based in your values, and demonstrates a deep awareness of the scholars, research initiatives, and impact that make the institution what it is.