char[,] grid = new char[3,3]; internal void Move(int position, char player)

// declare a grid
char[][] grid = new char[3][3];

// declare a function that takes in two inputs and returns no value
// use void because it returns nothing
void Move(int position, char player) 
{
  // if position is not valid, return
  if (position < 0 || position > 9)
  {
    return;
  }

  // find the row and column
  int row = position / 3;
  int col = position % 3;

  // update the grid
  grid[row][col] = player;
}