Use dashes to insert a comment or to highlight material within a sentence.

The pleasures of reading itself — who doesn’t remember? — were like those of Christmas cake, a sweet devouring.

—Eudora Welty, “A Sweet Devouring”

A single dash can be used to emphasize material at the end of a sentence, to mark a sudden change in tone, to indicate hesitation in speech, or to introduce a summary or an explanation.

In the twentieth century it has become almost impossible to moralize about epidemics — except those which are transmitted sexually.—Susan Sontag, AIDS and Its Metaphors

In walking, the average adult person employs a motor mechanism that weighs about eighty pounds — sixty pounds of muscle and twenty pounds of bone. —Edwin Way Teale

Dashes give more emphasis than parentheses to the material they enclose or set off. Many word-processing programs automatically convert two typed hyphens with no spaces before or after into a solid dash.