The radio show “This American Life” profiles what’s happening on Wall Street with “Another Frightening Episode About the Economy.”

This is must listening for figuring out what’s behind the current stock market sell-off. It describes the workings of “credit default swaps,” which financiers used originally to insure their investments in corporate bonds and make sure they were covered in case the bonds ever defaulted.

Because corporations are highly motivated not to default on their bonds — their bond ratings determine whether they can borrow money to keep their doors open and the lights on — buying insurance against bond default seemed like taking out fire insurance on something that had no reasonable expectation of ever burning down. You know, risk-free.

It works like this: The bonds exist in a world of their own and anybody can buy CDS (credit default swap) as an insurance policy against these bonds never being repaid. In the regulated insurance business, the only person who can insure your house is you. Now imagine if it were possible for all your neighbors to buy fire insurance on your house as well: this is how the unregulated CDS market works.

Why on earth would your neighbors want to buy fire insurance on your house that allowed them to be paid the house’s value if it burned down? Well, say your next-door neighbor saw your toddler playing with fire in the backyard and all the sudden he knows there’s a firebug living on the premises and the fire risk is much higher than anybody else believes. He’d love to go to your insurance agent and say “I’ll pay you a thousand bucks right now if you write me a fire insurance policy so I get paid if the house burns down.”

Your insurance agent knows nothing about the firebug toddler and figures this is the easiest thousand bucks he’ll ever make. It’s so easy that he might sell the premium to somebody else down the line and pocket a commission, or just to make sure he doesn’t get burned, he buys an insurance policy from another agent agreeing to pay him if your house burns down.

This set-up would work fine — all your neighbors and their insurance agents could take out policies on the homes in your neighborhood based on their perceptions of how likely it is that your house will burn down. But imagine what happens if your home actually burns down: now all these insurance agents who’ve taken their easy thousand bucks owe all the neighbors the full value of your home. (Won’t your neighbors cash in big time? Not necessarily: they were buying and selling policies on other homes and now that a house has actually burned, nobody know what those investments are worth anymore).

Remember when Lehman Brothers, the giant investment bank, failed a few weeks back? Well, its bonds were insured by something like $400 billion worth of credit default swaps. When Lehman went bankrupt, its bonds went into default. And now, all the big banks on Wall Street and a bunch of big hedge funds are on the hook for most of that $400 billion, and that’s why they’re not lending to anybody: they’re hoarding cash because they own CDS contracts obliging them to pay anybody who bought insurance on Lehman bonds. An auction will be set up later this week to figure out who gets paid.

Later this month, another auction will settle CDS contracts on Washington Mutual’s defaulted bonds.

How did all these Wall Street wizards get themselves into this jam? Well, they wanted to hedge their risk: if they had one CDS contract requiring them to pay a million dollars if a bond defaulted, they made sure they had another CDS contract paying them the same amount if a default happened. All the Wall Street CDS players traded these CDS contracts back and forth and earned a profit or booked a loss depending on what the market thought the CDS contracts were worth.

Everybody partied like mad till the unthinkable happened: actual defaults on actual bonds. Then it was game over because all these CDS contracts were connected in a chain no stronger than its weakest link. A small number of defaults set off a chain reaction that is spreading havoc on Wall Street as people confront the obligations they’ve taken on in these CDS contracts. If you owe billions you don’t have — and can’t raise — you are insolvent.

It’s going to be a messy few months while all this gets sorted out. Hopefully there won’t be any more massive bank failures to pour gasoline on the blaze before the Fed throws enough money at the financial system to put the fire out.