If you want to get really deep, try playing around with Classic Mac Resources. Now, Classic Mac really doesn't use file extensions except to indicate when a file has been compressed (sit) and hex-archived (hqx) or whatnot. All of the details that are determined automagically by Windows by the file extension are embedded in the resource fork in a Mac environment. So, in this case, the icon graphics, how the program should be executed, and even executable code can be embedded in the file descriptor. Why? Well, on my Windows Desktop, every time I install a new app that can view graphics files, all of my graphical programs get into a war over which is the default. The same with any audio or video file, so I'm constantly having to reset my defaults and run into many different consistency problems when I'm trying to open files for browsing. With the Resource Fork, you can determine how a program is opened by editing the Fork. It won't default to anything other than the app you told it to default to. Also, in Windows, I have my icons constantly changing to the viewer's default. In Mac, the icon of a graphics file is often a thumbnail of the graphic, allowing me to immediately determine its content. I think that's just neat