A clause set off by em dashes is like dropping underwater while swimming breaststroke — just a quick dip before popping back to the sentence’s surface. A parenthetical clause is more like diving down to the pool bottom to pick up a coin. And a footnote is a full-blown scuba dive — you have strapped on equipment and left the surface behind and you had better, after going to all that trouble, see something interesting down there. from (Let Us Out of This Clause) by Ben Dolnick via The New York Times