Your friends want you to do something you've decided you don't do — copy off someone's homework, say, or sneak a look at the answers before a test. You've held this line before. But today they've got a new angle: "It's just this once. It's not a big deal. It's not like you do it all the time. One time doesn't count." It's tempting, because it sounds so reasonable. Just once. Where's the harm in once? You can feel your firm rule getting softer, bending around the word "just," as if a single exception couldn't possibly matter.