Interfaces often rely on metaphors to help users understand and learn the application. Probably the most well-known metaphor is the Xerox PARC -developed desktop metaphor that Apple acquired for the Macintosh computer from Xerox in exchange for Apple stock. Metaphors generally make it easier for users to operate programs, but badly used metaphors can hinder an application's usability.
When developing interfaces that use elements based on real-world counterparts, don't imbue the interface with properties that aren't associated with the real-world item. Up until Mac OS versions 8 and 9, the Macintosh was plagued by a very illogical primary method of ejecting diskettes and CDs: you had to drag them to the Trash. This didn't make any sense to casual users, and people would often think that dragging a disk to the trash would erase the disk. This metaphor was flawed due to a programmerism: deleting files is, in a sense, a form of putting something away, and ejecting disks is another. This wasn't at all obvious, though. Later Mac OS versions have corrected this problem by making the Eject command the primary method of ejecting disks. Mac OS X takes this a step further: when dragging the icon of a disk, the Trash icon in fact morphs into an Eject icon. Dragging the disk onto the Eject icon safely ejects the disk, as expected.
It rarely makes sense to make a metaphor-based interface act exactly like the real-world equivalent. Concentrate on the convenient parts of the real-world item and downplay the inconveniences. For example, it wouldn't make sense for a word processor to require the user to use an eraser tool to slowly and painfully erase typed text. Likewise, it wouldn't make sense to make emptying the Mac OS Trash or the Windows Recyble Bin require taking the trash out.