"You're going to love these workout pants," said the sales clerk wrapping my purchases. "They have a lot of stretch in them so they're perfect even when you're having an 'I feel fat' day."

I thanked her and left, pondering her words all the way home. I knew what she meant by an "I feel fat" day (I bought the pants, didn't I?), and yet it seemed a really strange concept. Since when did "fat" morph into an emotion like "happy" or "angry"? What does it mean to "feel fat" and why are some days so given over to that feeling that they're consumed by it?

Like St. Patrick's Day or Independence Day, fat is important enough to get a day named in its honor ‑- and it doesn't just come once a year. Get a bad shock from the number on the scale? Can't fit into a favorite pair of pants? How about a snide comment from a relative or colleague? That's all it takes to turn an ordinary Monday or Tuesday into IFFD (I Feel Fat Day).

Carrying extra pounds is a necessary but not exclusive prerequisite to an IFFD. To have an IFFD, one must also carry an element of shame. A good IFFD is not complete without tears, self-loathing and a frantic scramble for concealing clothes. I know plenty of people (most of them male) who have fat without the shame. These men and women don't feel they can't leave the house without their special fat day pants. They don't feel unlovable, undeserving or invisible to the opposite sex. In fact, many of them are certain that this month's flavor of supermodel would find them irresistible if only they could wrangle an introduction. Needless to say, these people don't have IFFDs. They've got plans to lose the excess weight (sometimes more like a vague intention than a plan, but that's another story), and that's all there is to it.