It's Saturday afternoon, and you're standing in line at a movie theater. A bunch of kids from your class at school stand all around you -- kids you've known all your life. As you grew up together, you and these friends were always about the same size. Now all of a sudden the girls are taller than the boys.

You don't like to say anything about it, but you're beginning to wonder when the boys are going to catch up.

It's normal for girls to be taller than boys at certain times in their lives. That's because females start their growth spurt earlier than males do. For girls, this period of fast growth begins sometime between ages 9 and 16. Around age 12 is the most common time. For boys, the growth spurt begins between ages 11 and 18. Around age 14 is the most common time.

You don't have to be a math genius to figure out that there's a two-year gap in there. When girls start growing at age 12, boys are still in the slow stage of childhood growth. It can make for some awkward feelings for both boys and girls.

Human beings enter their stages of growth at different times, but we all go through similiar patterns. Before we're born, and while we're babies, we grow very fast.

If you've ever seen a baby when it was about 2 months old, and then again when it was 1 year old, you have seen fast growth in action. During the first year of life, infants grow half again as tall as they were at birth, adding about 10 inches to their height. They sprout another 10 inches between the ages of 1 and 4. Next, growth slows down. During the school-age years, kids grow more slowly, about 2 or 2 1/2 inches each year.

Each person grows at his or her own pace, however. It's improtant to remember that when you read numbers about growth on charts at your doctor's office, or in newspaper articles like this one. You may grow faster or slower than the "average" person. But that doesn't mean there's anything wrong with you.

Whenever your growth spurt starts, you need to take good care of yourself. The rapid changes in your body can make you tired and irritable. You may find it hard to concentrate in school. You may find that little things upset you more than they used to.

You need additional rest and lots of good food during this period of your life. Your body has work to do; if you're a boy, you'll grow somewhere between 4 and 12 inches in two-year period. If you're a girl, you'll grow almost as much.

During the growth spurt, glands in the body produce substances called hormones. Hormones are chemical messengers that tell cells to grow and change. One gland, the pituitary, produces the hormones that control how fast you grow, and when the spurt stops.

At the time of the growth spurt, other hormones bring other changes to the body. Rapid growth takes place at the same time that children enter adolescence. Boys' voices deepen. They begin to grow hair on their faces and bodies. Girls also begin to grow more hair on their bodies, and they begin to develop breasts. The transition from childhood to adulthood has begun. Tips for Parents

If your child isn't growing at the same rate as other children, there usually is no cause for alarm. He or she may just have a slower biological clock. Your child's chronological age may be 15 -- but biologically he may be 12. In time, he'll probably catch up.

If you become very concerned, however, you can consult a pediatric endocrinologist and have studies made of the bones in your child's hand and wrist. By comparing these X-rays to growth standards established by the National Center for Health Statistics, the doctor should be able to determine what your child's growth potential is.

Four common-sense rules can help your child attain full physical potential. These recomendations come from the Krogman Growth Center at the University of Pennsylvania Hospital in Philadelphia.

* Make sure your child gets enough sleep. Lack of sleep can actually slow the pace of growth during the adolescent spurt.

* Encourage your child to get regular exercise.

* Provide your child with a balanced diet -- paying particular attention to proteins, which provide the building blocks for muscle and skin.

* Recognize and deal with emotional problems your child may experience. Extreme stress can interfere with physical growth.