The most recent short interest data was recently released by the NASDAQ for the 05/15/2015 settlement date, and Fastenal Co. ( FAST) is the #4 most shorted of the S&P 500 components, based on 17.47 "days to cover." There are a number of ways to look at short data, but one metric that we find particularly useful is the "days to cover" because it considers both the total shares short and the average daily volume of shares typically traded. The number of shares short is then divided by the average daily volume, to express the total number of trading days it would take to close out all of the open short positions if every share traded represented a short position being closed.In the case of Fastenal Co. ( FAST), the total short interest at the 05/15/2015 settlement date was 38,199,664 shares, which compares to the average daily trading volume of just 2,186,796 shares, for a "days to cover" ratio of 17.47. When short sellers eventually cover their positions, by definition there must be buying activity because a share that is currently sold short must be purchased to be covered. At the present levels of short interest, if from this point forward every single FAST share traded represented a short position being closed, then at the average daily volume of 2,186,796 shares it would only be during the 18th trading day that every short position would be closed. So it would stand to reason that should some unexpectedly good news come out, and short sellers did not have 18 days of patience but instead wanted to cover their short positions very suddenly, that situation could result in sending the stock higher until the higher price produces enough sellers to generate the necessary volume to close out those positions quickly.