procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
var
  xpos, ypos: integer;
begin
  xpos := x div 100;
  ypos := y div 100;
  with form1.Canvas do begin
    rectangle(100 * xpos, 100 * ypos, 100 * (xpos + 1), 100 * (ypos + 1));
  end;
end;