Whitson Gordon at Lifehacker just revealed the most exciting thing I’ve ever learned about Mac OS X: the secret to opening two instances of the same application simultaneously.

Unlike Windows, Mac OS X normally only opens each application one time. Then as you try to open new files the application just creates a new window for each one. This is ordinarily no problem at all — you can still have as many PDFs open in Preview as you want; there will just be only one Preview icon in the dock or in the ⌘→ switcher.

But what if you want to open the same file twice? That’s not possible! And what about applications like VLC that only open one window at a time? There, each new video you start playing replaces the previous one.

Opening a file with open -n changes all that. And my life is now happier for it.