Use dashes--sparingly--to indicate abrupt shifts in thought and to set off or emphasize appositional or parenthetical elements or interjections. In most cases, use commas or parentheses instead.
Although we have made these comments with specific
reference to water--only because of our familiarity with
water--all pure substances exhibit the same behavior.
In typewritten documents, use two hyphens (--) with no spaces between or around them to form a dash.