双子 (Twins) are two offspring produced by the same pregnancy. They can be classified into two main types: identical twins, who develop from a single fertilized egg that splits into two embryos, and fraternal twins, who develop from two separate eggs fertilized by two different sperm cells. Identical twins share the same genetic material, while fraternal twins are genetically similar like regular siblings. Twins can occur in various species, including humans, and are often studied in fields like genetics and psychology. The occurrence of twins can be influenced by factors such as family history, maternal age, and certain fertility treatments. In many cultures, twins are considered special and may have unique cultural significance.