Wow. What’s the secret to such fantastic profitability?

1. Print money

2. Buy T-bills

3. Collect interest

4. Profit!

But hang on to your seats, folks. We’re not out of the woods yet:

If the price of Treasury bonds or mortgage-related securities issued by Fannie Mae and Freddie Mac were to fall in the years ahead, and Fed leaders decided they need to drain money from the financial system by selling off some of their portfolio, the central bank would lose money. “If they do enough asset sales and rates go high enough, that could eat into future profits pretty substantially,” said Michael Feroli, an economist at J.P. Morgan Chase.

The Fed has increased its balance sheet from roughly $500 billion to roughly $1.8 trillion over the course of the crisis, with a lot of that taking the form of Fan/Fred toxic waste. But what are the chances the central bank will take a loss on that stuff, right?