No puedo editar StringGrid

[Bael_Balzac]

goEditing esta en True
goTab esta en True

No puedo editar las celdas en tiempo de ejecucion

Código Delphi [-]
unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, Grids;

type
  TForm1 = class(TForm)
    StringGrid1: TStringGrid;
    procedure StringGrid1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.StringGrid1Click(Sender: TObject);
  begin
       with StringGrid1 do
       begin
       Cells[0,0] := 'Nombre';
       Cells[1,0] := 'Apellido paterno';
       Cells[2,0] := 'Apellido materno';
       Cells[3,0] := 'Nota Examen 1';
       Cells[4,0] := 'Nota Examen 2';
       Cells[0,1] := 'Pablo';
       Cells[1,1] := 'Garcia';
       Cells[2,1] := 'Martinez';
       Cells[3,1] := '40';
       Cells[4,1] := '36';
       Cells[0,2] := 'Maria';
       Cells[1,2] := 'Sanchez';
       Cells[2,2] := 'Palazon';
       Cells[3,2] := '50';
       Cells[4,2] := '60';
       Cells[0,3] := 'Carmen';
       Cells[1,3] := 'Perez';
       Cells[2,3] := 'Guillen';
       Cells[3,3] := '70';
       Cells[4,3] := '60';
       end;
end;

end.