In stock trading, you measure stock market activity & liquidity by volume. In options trading, there are two measurements: Open Interest & Volume.Unlike in stock trading, whereby there is a fixed number of shares to be traded (i.e. number of outstanding shares), in options trading, new option contracts need to be created when a trade is placed and there is no existing contract yet. When a new expiration month initiated, there is no open interest because there are no option contracts being traded for that month yet. As trading builds up, open interest will also increase. Now, what is Open Interest?Open interest is the total number of option contracts that are still open (i.e. have not yet been exercised, or have not been closed out by an offsetting transaction, or have not expired).Open...