What is the capital of the United States?

Why this is the answer

Washington, D.C. is the seat of the federal government, and the story of how it got there explains most of what is unusual about it. When the Constitution took effect in 1789, Congress met in New York City. Lawmakers from the North wanted the permanent capital in their region, and lawmakers from the South wanted it in theirs. The bargain they struck, the Compromise of 1790, placed the capital in the South, and in exchange the North was excused from paying the debt it still owed from the Revolutionary War. George Washington chose the exact site on the Potomac River, between Maryland and Virginia. Philadelphia hosted the government in the meantime, and after ten years there the move to the new city came in 1800. The framers had planned for a capital like this. Article I, Section 8 of the Constitution gives Congress exclusive lawmaking power over a district no larger than ten miles square, ceded by the states, to serve as the seat of government. That clause is the reason the District of Columbia is not part of any state and is run under federal control. Presidential elections work differently there too. The Twenty-third Amendment, ratified in 1961, gave the District its own electors, capped at the number held by the least populous state. On interview day the risk is less about forgetting than about saying something close but wrong. New York and Philadelphia both served as the capital in the early years, so they can sound right if you are nervous. Washington is also the name of a state, so give the full name, "Washington, D.C.," rather than the first word alone. One quick way to anchor it: the city is named after the first President, the same man who picked the spot on the Potomac where it would be built.