Twins

It's amazing to me how some children possess either their mother or father's characteristics so closely, and others look so completely different than their parents. What's even more amazing to me, is the ability to have either identical or fraternal twins and be able to see the differences in the two children on multiple levels. 
The biological variatons with twins is hard for many inidividuals to understand, but learning about the difference in meiosis and the physiology aspect of how twins and offspring are created is so interesting to learn about.

My best friends nephews are fraternal and seeing them develop different personalities and characteristics is so amazing, knowing that they are twins, yet born from two separate eggs and fertilized by two separate sperm is something that I find extremely interesting.