Teen-agers don't diet like other, normal people. I'm learning this first-hand from my 13-year-old daughter, Ann, who announced morosely two weeks ago that she is FAT.

Well, as my husband said, no one could deny that she is a bit chubby, if not outright obese. And so, reminding myself that the new theory is to think thin, I asked the milkman to leave me skim milk instead of whole. That was the first step, and not altogether a successful one.

She doesn't like skim milk. It tastes funny. It looks blue. So does my face by the time I finish listening to her complaints. She finally compromises. She uses whole milk on her cereal, and buries the taste of the skim for drinking with three tablespoons of sweetened chocolate. I try to explain, patiently, that is known as defeating the purpose. It is also known as inviting your skin to erupt in as many bumps as the moon.

She nods understandingly and looks through me. I give up and move on to the next step.

I submitted cottage cheese for her baked potato with dinner that night. Tears came to her eyes. I took the cottage cheese and gave her my potato. There are less than 100 calories per potato, she smilingly explained, and spooned the sour cream right over the lump of butter. I attacked the cottage cheese with a vengeance.

On her way to bed that night, she gaily called out "Remember, Mom, no more sandwiches for lunch." She's always hated sandwiches anyway, so that didn't surprise me. I filled her Thermos with cottage cheese. Still optimistic, and trying to be as helpful as possible, I got up early the next morning and fixed her what I thought was an attractive array of raw and peeled carrots, celery, etc., to go along with the cottage cheese.

She came in from school looking injured. "I'm starved," she announced, "and, boy am I sick of cottage cheese."

The next morning, she requested soup.

I was getting a little less helpful by now. I just opened a can of chicken noodle, heated it up, and tossed it into the Thermos. No garnishes this time.

She came home from school that day with half a Thermos of cold noodles. She had spent the lunch hour picking the noodles out of the soup, because they are fattening, she explained sulkily. And, according to her, she was so starved after such a skimpy lunch that she had had to spend 20 cents of her own money for an ice cream sandwich.

She was still starved. Could she make a cake? I wondered if dieting could cause dementia.

"A CAKE???"

"Well," she explained, "I think I'll just frost the top and the middle, and not the sides.Then it won't be so fattening."

I threw up my hands, and out went the diet dinner dish I had spent most of the afternoon preparing with them.

"Sure," I replied blithely, "make two cakes. Then you'll have one for tomorrow, too."

She couldn't imagine why I was uspet.

I spent dinner watching her help herself to minuscule portions of chicken, vegetables and salad - washed down, of course, with her super-chocolatey skim milk. She was saving herself for the cake.

When my husband started to say something to her, I kicked him under the table and motioned to him that we would discuss it later.

It was a short discussion, I said it was obvious that the child wasn't very good at dieting. What she really needed was more exercise. The solution was for him to take her with him on his weekend hikes. He said "No."

I had to get kind of mean with him. I reminded him that any tendency she had to be overweight was inherited from his side of the family.I was from a family of beanpoles. He finally agreed to try it the following Saturday.

It might even bring a new closeness between them, I told myself. What it really brought between them was a stony silence that lasted for days.

My husband finally told me what had happened. According to him, she had twisted her ankle about a tenth of a mile along the path, then talked of nothing else for the remaining three and nine-tenths miles of the hike.

I found this hard to believe. When I pressed him, he did admit that she had talked a lot about how hungry she was, too. He then officially resigned as her exercise coach.

So it was back to shaving off more pounds with fewer calories.

Another week and two pounds later, i was about to despair. I was also about told declare bankruptcy (two boxes of $3.95 weight-reducing candy in four days), when a modern-day mircale occurred in the form of a "really cool 10th grader," named Phil.

It seems this Phil offered her a hot chocolate at the local ice rink last Friday night. She refused, with the excuse of her diet. The cool 10th grader did us all an enormous favor by telling her that she was just right.

My husband rather basely asked her if this Phil was stocky. She just smiled aloofly, and dropped the diet like a hot potato (less than 100 calories, remember?)

I heaved a sigh of relief. The next chubby child I have is now only 3 years old. I'm wondering if 10 year is long enough to rest up for another Terrible Teens diet.