You've probably already played Tic Tac Toe. Isn't it a very simple game? That's what the majority of people believe. But if you really think about it, you'll realize that Tic Tac Toe isn't as simple as you think! Tic Tac Toe (like many other games) requires you to look ahead and try to predict what the person playing against you will do next.
The Os is your adversary, and you are the Xs. On your turn, click anywhere on the grid to put an X there. Get three consecutively before your rival in order to win. Try to line up four squares on the 5x5 grid for an added challenge. Switch to hard mode if things are still too simple for you. On the same device, you have the option of playing against the computer or a friend.