Tic-tac-toe
Tic-tac-toe, also called noughts(/cricles) and crosses or X's and O's(/0's(/zeroes)) is a game for two people that only requires pencil and paper, although you can also use a board with tokens.
The board (which can be a drawing on a paper or other place) consists of a square that is crossed by two vertical and two horizontal lines, so in total there are three small squares left in the top line, another 3 in the center line and 3 more in the bottom line.
Each player takes turns drawing a cross or a circle in a square (or placing tokens of different shapes or colors) The winner is the one who manages to place three equal pieces in a row, either vertically, horizontally or diagonally.
Tic-tac-toe Media
Incidence structure for tic-tac-toe
Optimal strategy for player X if starting in upper left. In each grid, the shaded red X denotes the optimal move, and the location of O's next move gives the next subgrid to examine. Only two sequences of moves by O (both starting with the center, top-right, left-mid) lead to a draw, with the remaining sequences leading to wins from X.