<prompt> Human: Reading the Big Short book and came across the following passage: He had now a tactical invedtment problem. The various floors, or tranches, of subprime mortgage bonds all had one thing in common: the bonds were impossible to sell short. To sell a stock or bond short, you needed to borrow it, and these tranches of mortgage bonds were tiny and impossible to find. You could buy them or not buy them, but you couldnt bet explicitely against them.Update: thank all for your claryfing answers! That u can sell something u dont own its a crazy idea, I guess this what W street came up with to make more money out of these bonds. Scary!Edit: corrected typo - own instead of know  <response>: Basically, it says it in that passage. To Sell short means you borrow the stock off someone, sell it to someone else, and then buy it back later and return it to the person you borrowed it off. If the stock has gone down in that time period you make money. If it has gone up you loose money. 