With Treasury Secretary Jack Lew making clear earlier this week that Oct. 17 is the drop-dead deadline for Congress to raise the debt ceiling or run the risk of default, the timeline for what is already shaping up as a massive political showdown is set.

We will write lots (and lots) about the political calculations involved in the debt ceiling fight. But, before we do that, let's get back to basics. As in, what is the debt ceiling and why does American have one?

Karen Tumulty, Post national politics reporter and witty Tweeter, answers those questions in this segment for "In Play". Watch!

The U.S. is the only big country with a debt ceiling. The Post's Karen Tumulty explains why. (The Washington Post)