" When investors go short, they are hoping to profit from a stock decline by selling borrowed shares and then buying them back later for less money. Shorting is profitable when stocks fall, but it can be painful when they rise. If you short a stock at $50, and it goes to $70, you just lost $20, or 40% of your bet. Worst of all, unlike buying a stock, shorting can expose you to unlimited losses: while a stock can only fall to zero, it can – theoretically, at least – rise forever. Just ask the poor sap who bet against Apple five years ago. Think of it as the flip side of buying."