Or the big boogy man from the crisis, credit default swaps. They're complex financial basically insurance that you can take out on any asset, even one you don't own. Many institutions saw the chance of these securitized mortgage assets failing as so remote that they'd sell credit default swaps for fractions of a penny on the dollar on their coverage. They were cheaper and more predictable then shorts and when the crisis hit if you held them you made a killing. Of course a lot of that killing came on the back of the U.S. taxpayer. The number of credit default swaps that they sold were what almost killed AIG, so the government stepped up and covered almost $14 billion of their losses.