When an investor believes a stock is overvalued and will soon drop in price, he might decide to "short" it. First, he borrows an amount of the stock, and then sells it. He waits for the stock to tank before buying back the same amount of shares at a deflated price. After returning the shares to his lender, he pockets the difference--unless any one of several hard-to-predict variables interferes, and the stock fails to drop.