WHAT IS THE MEANING OF EUROPEAN?
In addition to CALL options there are PUT options. They give you the option to sell a stock at a fixed time for a given strike price. So even if you don't own the stock at the time of exercising the option, if the value of the stock is lower than the strike price, you just go ahead and buy the stock at market value and sell it right away - again for the guaranteed strike price - and you made money! Again - also put option have a price that have to be taken into account.
You can see the payoff of a put option in the pop-up window.