... TextEdit document icon, for example, TextEdit opens 5.6. How Documents Know Their Parents Every operating system needs a mechanism to associate documents with the applications that created them. ... them, however, choose Finder Preferences, click the Advanced button, and then turn on "Show all file extensions." Now examine a few of your documents; you'll see that their names ... Safari, and so on. How does Mac OS X know which of these programs to open when you double-click the file? Fortunately, you can decide. You can reassign a document (or all documents of its kind)...