Your best friend asks you if you want to play a game, and you reply, "Sure!" She grabs a piece of paper and a couple of pencils. After marking two vertical lines on the paper, she crosses them with two horizontal lines to make a three-by-three grid with nine squares. When she marks an X in the center square, it's game on!
What are you playing? That depends upon where you live. If you live in England, you probably call it noughts and crosses. If you're in Canada or Ireland, you might call it Xs and Os. If you're in the United States, though, you're definitely playing tic-tac-toe!
No matter what you call it or how you spell it (tick-tack-toe is also acceptable), tic-tac-toe is an ancient game that billions of people have played over the course of history. While some experts believe the game dates back to the ancient Egyptians, others believe it evolved from an ancient Roman game called Terni Lapilli.
Terni Lapilli was played on a similar three-by-three grid. In fact, these grids can be found scratched onto all sorts of surfaces around ancient Rome. However, no markings have ever been found within the grids, leading historians to believe the game was played with movable pieces rather than by making markings, such as Xs and Os.
Experts believe the name tic-tac-toe came about sometime in the 1800s. Way back when, a popular game called either "ticktack" or "tic-tac-toe" was played by blindly throwing a pencil at a slate marked with numbers. Your score was tallied by counting the numbers hit with the pencil. This game is no longer played today, but its name survives nonetheless. Historians believe the name derived from the sound the pencil made when it hit the slate.
The modern game we know as tic-tac-toe is played on a nine-square grid created by drawing lines to create three horizontal rows and three vertical columns. With X going first, players alternate marking the squares with Xs and Os, trying to achieve three marks in a row either horizontally, vertically, or diagonally.
Tic-tac-toe is interesting to mathematicians, because its small grid and simple game play can be used to teach simple mathematical principles, such as probability. For example, did you realize that there are 362,800 unique ways to place Xs and Os into the grid?
Of those combinations, 255,168 of them are possible winning combinations. However, if you eliminate all the symmetrical combinations, there are only 138 unique winning combinations. With this few winning combinations, tic-tac-toe is a relatively easy game to play, making it popular with children. Most players quickly learn that two good tic-tac-toe players will settle into playing to a draw repeatedly.