Look into the Mirror - Stern - Bog

Technically, twins are two children born about the same time from the same mother. Twins are more common than you'd think-about once in every ninety births. Twins can be either fraternal or identical. The former develop when two separate eggs are fertilized at about the same time. Identical twins are produced as the result of a single fertilized egg splitting in two. Each half develops as a separate fetus, but with identical genes. There are several additional twin anomalies: including so-called mirror image twins, who are identical, but in reverse. Ryan was the yang to Sean's ying. Even though they were identical, they were opposite. Ryan was right-handed, but Sean was left-handed. Ryan was an excellent student-not so with Sean. They were, however, inextricably bound together which is not surprising. There was nothing one wouldn't do for the other.