Working with Delphi Tthread

This is the first time to try working with Threads, I'm trying to copy a directory using a Thread, so here is what I did (After I read this post):

unit Unit1;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, System.IOUtils, System.Types;

type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;
TMyThread= class(TThread)
  private
    Fsource, FDest: String;
  protected
  public
    constructor Create(Const Source, Dest: string);
    destructor Destroy; override;
    procedure Execute(); override;
  published
end;
var
  Form1: TForm1;
  MT: TMyThread;
implementation

{$R *.dfm}

{ TMyThread }

constructor TMyThread.Create(const Source, Dest: string);
begin
  Fsource:= Source;
  FDest:= Dest;
end;

destructor TMyThread.Destroy;
begin

  inherited;
end;

procedure TMyThread.Execute;
var Dir: TDirectory;
begin
  inherited;
  try
    Dir.Copy(Fsource, FDest);
  except on E: Exception do
    ShowMessage(E.Message);
  end;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
MT := TMyThread.Create('SourceFolder', 'DestinationFolder');
try
  MT.Execute;
finally
  MT.Free;
end;
end;

end.

When I click on the Button1 I get this error message:

Cannot call Start on a running or suspended thread

What's wrong here? I don't know much about threads,I even try:

MT := TMyThread.Create('SourceFolder', 'DestinationFolder');