The game takes place on the field of squares. A square may be empty ("dead" cell) or filled ("alive" cell). A live cell will remain alive, if it has 2 or 3 neighbors. It will die, if neighbors are less than 2 or more than 3. A new cell will be born, if this place has exactly 3 neighbors.

The important moment that is the most difficult for understanding: the change of generations occurs at once. The fact that some cell is going to die or to be born, doesn't affect the count of neighbors. In other words, the count of neighbors is calculated on the previous turn.

More formalized, imagine that you have the second field for cells that is empty. Fill a cell in this second field if the following conditions are satisfied:

• the cell in corresponding place of the first field is alive and it has 2 or 3 neighbors, OR
• the cell in corresponding place of the first field is dead and the number of its neighbors is 3.

When these conditions are not met, a cell remains empty. Do not fill anything in the first field while you are filling the second one. After you finish, the second field represents the pattern of the next turn.