Explain how is it possible to have twins that are not identical

Identical twins develop from the same egg and sperm, the embryos split and form identical twins, they are the same sex. But some are fraternal twins, they develop with two diferent sets of sperm and eggs by the same parents. They can be from diferent, this is like having diferent babies at the same time.

Fraternal (dizygotic) twins come from 2 different fertilized eggs that both grow in the mother's womb. They have different DNA (2 eggs from the mother, 2 sperm from the father) and are not identical, although they often look alike.

Identical (monozygotic) twins come from one fertilized egg that has split in two. These twins share the same DNA and are completely identical.