By Mark L. Chambers from "Mac OS X Lion All-in-One For Dummies"
Every Mac owner needs to know how to protect files and folders from accidental deletion or editing. By locking a file in Mac OS X Lion, you allow it to be opened and copied — but not changed, renamed, or sent to the Trash. Locked items appear in the Finder with a small padlock attached to the icon.
To lock or unlock a file, you have to have ownership of the file, which you should have if you’re the only administrator.
Warning
If you’re considering changing the ownership of a system-owned file, don’t do it. You could throw a serious monkey wrench into your Lion system.
To lock a file, follow this procedure:
1. Display the Info dialog for the item.
2. Select the Locked check box to enable it.
The Locked check box is in the General section of the dialog.
