Msgbox not Appearing Upon Opening Excel File

 Private Sub Workbook_Open() 
     msgbox "Hello" 
End Sub

Why isn't this working? I am sure this is the code where you open a workbook, a pop up msgbox contains "Hello" text will appear. For my case, it has no respond.

My filename is Book1.xls. Does the filename affect this?

I have copied this very code in Sheet1 tab.

Help.