Jeffrey Ewen, age 11, of Valparaiso, Ind., for his question:

WHAT ARE SIAMESE TWINS?

In very few rare cases, identical twins are sometimes born joined. A band of flesh usually ties them together at the hip, chest or abdomen.

Joined twins are called Siamese twins. The name came from a pair of famous joined Chinese twins named Eng and Chang who were born in Siam. They lived from 1811 until 1874.

In some cases, Siamese twins are simply joined by tissue through which the blood circulatory systems flow from one to another. Other times the two may share a vital organ. Today, if each child has his own vital organs, surgery often successfully separates such twins.

Craniopagus Siamese twins are those joined at the head. At the National Institute of Health in Bethesda, Md., a 4 month old pair of craniopagus twin girls were separated in 1956. Both survived the surgery and developed into normal adults.