I have never discovered why it is that essay examinations at colleges around the country are written by students in variations of the "bluebook": eight or 16 lined pages stapled into a light-blue cover.

It's true that this format helps professors to judge each student's work on content rather than appearance. But why do we use stapled books for exams, instead of loose-leaf paper, which we use the rest of the year? And why must they always be blue?I don't know. But I do know an urban legend in which a student uses the bluebook custom to obtain a top grade.

When I was a college student, professors distributed the blank bluebooks. Now, we generally require students to purchase their own at the campus store and bring them to the examination.

Either way, there are always a few extra bluebooks on hand during an exam. These allow for the genius whose essay fills several books, or the con artist who skips lines and pages or writes in a huge script so as to make a skimpy answer seem longer.

As the legend opens, a college student is at a final examination with a blank bluebook open before him. At first he seems unable to answer the essay question. After a few moments, however, he uncaps his pen and begins to write in the book.

What he writes is a letter to his mother. He opens with an apology for not having written recently, explaining that he has been studying so hard for finals that he hasn't had time to write. He goes on to say that he crammed so thoroughly that he was able to finish the exam with time to spare. Now he's taking advantage of the remaining time and a second bluebook to write this overdue letter.

The student turns in this bluebook at the end of the exam period. Then he hurries home, consults his textbooks and class notes, and writes a fine essay answer in a second bluebook. He immediately mails this bluebook to his mother.

Before long the student gets a telephone call from the professor, who is puzzled. Why, the prof wonders, was there a letter to the student's mother among the exam bluebooks? And what happened to the student's essay?

The student explains, in a voice full of dismay, that he must have accidentally switched the two bluebooks as he left the classroom. When asked how he could have been so foolish, the student pleads with the professor to call his mother back home and verify the story.

Sure enough, when the professor calls the student's mother three days later, she says that she has indeed received a bluebook essay from her son. And yes, the envelope was postmarked the day of the exam. She was a little puzzled herself, she says.

The prof and the mother exchange bluebooks, and the student's super essay earns him an A in the course.

The legend is widely known on college campuses, but never attributed to anyone the storyteller knows. It's the usual friend-of-a-friend story in folklorist's parlance, a FOAF.

Given the prevalance of the bluebook custom at colleges, it would be quite possible for a student to play such a trick. Or a student might fill a bluebook with answers beforehand, then somehow smuggle this book into the pile of exams.

But in my years of college teaching I've never uncovered an act of cheating based on a switch like the one above. Nor have I met anyone who has.

I urge students reading this column not to try this ploy in their own examinations. Most professors have heard the story before, and the rest are likely to figure out on their own what the student is up to.

Besides, you wouldn't want to make your mother an accomplice, would you?

Jan Harold Brunvand is the author of "The Choking Doberman," a collection of urban folklore. Send your questions and urban legends to Prof. Brunvand in care of this newspaper.

(C) 1988 United Feature Syndicate Inc.