In an example from Steve Foerster, finance professor at Western University's Ivey Business School, say you think a stock trading for $100 is overvalued, because you think its shiny new product isn't any good - it's worth just $50 for a share. You can borrow the stock, sell it at $100, and plan to buy it back later at the lower price -- pocketing the $50 for yourself. But the stock market is never a sure thing. If the stock instead goes up in price, it can lead to losses for a short-seller. If the short seller has to buy the stock back at a higher price instead of their expected lower price, it can drive up the stock price even further, creating what's known as a "short squeeze."