Why do airlines offer the best fares if you agree to stay over at your destination on a Saturday night? No, it’s not because they care what you do in your spare time. It’s just their way of separating the wheat from the chaff, or the business traveler from the family guy.

Airlines make the bulk of their money on business travelers, the men and women whose companies send them off to remote destinations on a moment’s notice. Business travelers aren’t going to put off their trips until the rates get lower; they needs to get where they’re going, and since the journey is on the company tab, price is not as much of an object. So, with such a nice profit margin on Mr. and Mrs. Nine to Five, the airlines can afford to offer cheaper rates to pleasure travelers, those who might choose to drive a few hundred miles if they can’t get a decent fare.

But reservation agents can’t exactly ask each caller “is this trip for business or pleasure?” So they go by the basic rule of thumb, which is that business travelers typically travel only during the week, and rarely stay overnight on a Saturday while “on the job.” Those who are jetting off for fun or a vacation are more likely to wrap their trip around a weekend, and so a Saturday night stay is more typical.

Don’t feel like Corporate America is getting ripped off by paying higher rates, however. Those business travelers usually earn plenty of free trips along the way via their frequent flier miles.