A short squeeze takes place when short-sellers scramble to cover their bearish positions on optimism or any positive catalyst, driving the price of stocks up sharply. The metric for measuring short-squeeze opportunities is the short ratio, which is the number of days it would take the short-sellers to cover their positions based on recent average daily volume. Citigroup isn't the only stock that has short-covering upside potential. With this in mind, Stockpickr has compiled a portfolio of the top S&P 500 short-squeeze plays. To read more, visit Stockpickr.com. Who's on Stockpickr Answers? David Peltier will be on Stockpickr Answers on April 9 to respond to investing and trading questions posed by members of the Stockpickr community. Not a member? Join the Stockpickr community today -- free.