Mechanical Monday: The Dash

No, no, not the sprint you do when you're at the grocery store, have one item, an are trying to beat the dude trying to sneak eleven items into the "ten only" line.  I'm talking about the dash you use in writing, when you want to set something apart—a part of the sentence that's important and connected—but that's also an aside:

I went to Perth—thinking I'd find love—but all I found was fish.

The thing I've noticed about the stuff in between the dashes (when it's written in the format...