America’s debt now tops $16 trillion, and the meter never stops running. The national debt increases by about $35 million an hour, and around $2 billion every 24 hours.

As the country’s debt skyrockets, politicians and pundits are debating how to get it under control. If nothing is done, we could be $17 trillion in the hole later this year.

But for the first time in six years, the federal government said it would make a small down payment on the national debt – about $35 billion worth. They say higher tax receipts and recent spending cuts helped raise the money.

Which brings us to the topic of this Just Explain It.

What exactly is the debt? And how long would it take to pay it off?

First, here’s some history.

Before President Reagan took office, the national debt was $1 trillion. By the time President Clinton left the White House, it reached $5.6 trillion. Eight years later, the debt had almost doubled. And today it stands at $16.8 trillion.

So… where did all of this debt come from?

Investors are learning - the hard way - that when something seems too easy, it almost certainly is.

For most of this year, the idea of riding stocks higher - even buying successive new highs in the U.S. indexes – has been viewed as a path to effortless gains, supported by central banks’ easy-money resolve and a resilient (if moderate) economic expansion.

Now, suddenly, a cocktail of worry is served - its ingredients global-growth fears, unclear central-bank intentions and a market tape that had been stretched far to the upside. The mixture is a bitter one, most dramatically spurring a 7.3% buckling of Japan’s overheated Nikkei stock index as well as a shudder of downside volatility in U.S. stocks.

Related: Bernanke Speaks: Can Anything Slow Down This Market?

The Standard & Poor’s 500 Index (GSPC) had been “melting up,” in the common description, climbing a gentle slope to a new record at 1669 Monday, up 17% on the year and 23% since mid-November, without so much as a 4% pullback.