"Even our parents mix us up sometimes," says Susan, a third- grader. Susan and her sister Pam are identical twins. They look so much alike that they can fool their friends and teachers about which one is which. If you know them well, you can tell them apart because Pam has pierced ears and Susan doesn't. Or is it the other way around?

"We don't look alike," says Tommy, who's in fifth grade, "but our teachers are always calling us by the wrong name anyway. They sometimes just call us 'twin,' which makes me feel really mad." Tommy and his brother Bobby are fraternal twins. You can tell they come from the same family, but they don't look exactly alike.

What's the difference between these two sets of twins? Why are some twins identical, and others not? It all started before they were born.

A human baby grows from a tiny egg cell, or ovum, inside its mother's body. To start the journey toward becoming a person, the egg must join with a sperm cell from the father. When the two cells join, they produce another kind of cell called a zygote.

Both the ovum and the sperm cell contain enormous amounts of instructions telling the baby how to grow. They are contained in chemicals in the cells. Together, the cells carry enough of these instructions to produce all the characteristics of a new human being.

This "recipe" for a new person is contained in tiny threads called chromosomes. An ovum has 23 chromosomes. A sperm has 23, too. Together, they make 46 -- exactly the right number to start building a person.

The chromosomes carry bits of chemical instructions called genes. Genes control many things about what each new human being will look like. Will it have red hair, or brown? Will it have an upturned nose? What shape will its earlobes be? The genes decide these things. Since a baby gets half of its genes from its mother and half from its father, it usually ends up looking like some combination of its parents. There are so many possible combinations of genes that each human being looks unique.

Wait a minute -- what about identical twins? They look alike because of an event that takes place soon after the ovum and sperm join forces. The zygote splits into two parts, each containing a complete genetic recipe for a new person. This is a pretty unusual event. Only one zygote out of about 250 splits into two, and develops into identical twins. Because the genetic instructions of each developing baby is the same, they will look identical, and will be the same sex. Fraternal twins, on the other hand, develop from two separate egg cells. They develop together inside their mother's body, and share the same birthday. They may both be girls, or both boys, or they may be one of each. Like ordinary brothers and sisters, they share family characteristics. But they weren't produced from the same genetic recipe. Fraternal twins aren't quite as rare as identical twins -- they happen in about one pregnancy in 150.

Sometimes there are three babies at a time, and they are called triplets. Sometimes there are even more. One woman last month had septuplets -- seven babies.

After twins are born -- whether they are identical or fraternal -- they grow into individual people. Some identical twins may have a hard time convincing people that this is true. Are there any pairs of twins in your school? Make an effort to treat them as separate and unique people rather than as "the twins." They'll appreciate it -- and you'll make two special friends.

Twins' parents can help make them feel like special people, too. "Sometimes my parents take me out by myself," says Pam. "We talk about things that interest me. Then the next week they'll do the same thing with Susan. It's neat."

"It's a lot of fun to be a twin," says Bobby. "But I do get tired of people thinking that because Tom likes swimming, I must like it, too. I like reading! We're both individuals." Tips for Parents

Being a twin can be both fun and stressful; so can being the parent of twins. If you're looking for support, guidance or just a place to say to someone who understands, "Will I ever get a whole night's sleep again?," contact the National Organization of Mothers of Twins Club Inc., 5402 Amberwood La., Rockville, Md. 20853, 460-9108. Marion Meyer, executive director of the national organization -- and the mother of 29-year-old identical girls -- will refer you to the nearest club. There are three in Northern Virginia, one in Montgomery County, one in Prince George's and one in the District. The clubs hold monthly meetings and periodic clothing and equipment sales. It maintains libraries about twins, parenting and twin research. "It's a place to find out there are other people who feel, 'Yes, I've worked twice as hard -- but I have three times the blessings," says Meyer.