SOS is a (usually two-player but may be more) game played with paper and pencil, typically played by children. It is similar to tic-tac-toe but with more complexity.
Before play begins, a grid of n × n squares (with n being 3 or more) is drawn. The first player to move (which can be chosen by, e.g., a coin toss) writes either S or O in one of the squares, whereupon it becomes the second player's turn. The second player has the option of writing an S or O in an empty square.
The object of the game is for each player to attempt to create (on their turn) the straight sequence S-O-S among connected squares (either diagonally, horizontally, or vertically), and to create as many such sequences as they can.
If a player succeeds in creating an SOS, that player immediately takes another turn, and continues to do so until no SOS can be created on their turn. Otherwise turns alternate between players after each move.
Keeping track of who made which SOSs can be done by, e.g., one player circling their SOSs and the other player drawing a line through theirs.
Once the grid has been filled up, the winner is the player who made the most SOSs. If the grid is filled up and the number of SOSs for each player is the same, then the game is a draw. If the grid is filled up and no SOS has been made, then the game is again a draw.
The game may well have begun spontaneously in more than one location by different people unrelated to each other, at different times. But one claim to the origins of this game is that it was birthed at Wairarapa College, in New Zealand, in 1947 by members of a year 9 class that had a games day. Some boys in the class were tired of playing "tic tac toe" and wanted something that would last a bit longer.
SOS is a game of skill and an abstract strategy game. It is also a combinatorial game (when played with two players). In terms of game theory, it is a zero-sum, sequential game with perfect information.