Let's just say that if when your mom was pregnant with you, she was pregnant with another embryo at the same time. Either identical or fraternal. And let's say that the two developing embryos fused together to make one whole person. How would you know what part of you is you. Like are you the left side or the right side. Or maybe you are just some veins and half the fingernails. Like how exactly would you know what part of yourself is really you and how would you determine what half you would be anyway. If you have no idea what I am talking about then do a Google search on human chimerism (sp?) It's fascinating stuff. I should sleep more.