The big investment bank folded over the weekend and got sold for pennies on the dollar. Basically, Bear was heavily involved in securities tied to the mortgage meltdown, and last week there was essentially a run on the bank. By Friday all of Wall Street was avoiding it like Superman avoids kryptonite.

To understand what killed the Bear, you have to understand — to the extent that this is possible for non-financial types — how hedge funds and derivatives speculators make money. The book about the failure of the hedge fund Long Term Capital Management explains that hedge funds make money by borrowing against securities with a leverage ratio far, far beyond anything the rest of us would ever dare to try.

To wit: if your house is worth 100,000 and you owe 100,000, that’s a 1-to-1 leverage ratio. But the assurance that you’ll pay it off — because you have a strong motivation not to default — means you could, in theory, borrow against the assurance of payback on the note, because that assurance is in essence a kind of collateral. You know, something with market value. Say you talked your bank into loaning you another $100,000, then you’d have a 2-to-1 leverage ratio.

Hedge fund managers, who have billions to play with and the best brains and computers in the business, can find ways to make money off that assurance of payback in ways that would make your head spin. A single security could have a 15-to-1 or even 30-to-1 leverage ratio. These seem like stupendously risky bets when a measly million dollars, say, has 15 million dollars borrowed against it. Thing is, in ordinary circumstances, there’s no realistic risk of having to pay back all these loans at the same time.

To visualize this, imagine the Flying Wallendas with their Human Pyramid: it had four guys on the base, two people in the middle and one on top. Now, imagine the pyramid upside down, with one very strong beefy guy on the bottom and everybody else balanced on his shoulders. Theoretically, such a balance is possible, but it’d probably take a computer and the world’s greatest acrobats to pull it off. A long as everybody keeps their balance, everything’s fine. But if anything highly unusual happens — like, perhaps, one of the biggest investment banks in the world collapsing — the pyramid falls and there’s blood under the Big Top.

Bear Stearns wasn’t a hedge fund, but it was in the hedge fund business, and it had billions in borrowings out there that it couldn’t pay off with cash on hand. Late last week, Wall Street turned on Bear with a vengeance, and everybody wanted what Bear owed them, and they wanted it now.

A bank or hedge fund can survive if it can get its hands on enough cash to survive till the panic subsides. That’s where the Fed came in, offering to stand behind $30 billion worth of Bear securities till Wall Street stops acting like cattle in full stampede mode.

Long-Term Capital Management was rescued by a deal in which the biggest investment Wall Street Banks chipped in a few billion apiece to provide a cash life line to the fund until the run on its securities ended. Bear Stearns refused to participate. Cosmic payback arrived over the weekend.

I can’t help wondering how many leveraged-to-the-hilt securities out there presume the continued existence of Bear Stearns, and what happens to them when it disappears. Then again, I’m not too sure I want to think about that.