For instance, I have a cousin who is now 19 yrs old. His mom was pregnant with him when she married my uncle. She was at a bar and had a one-night stand. She has no idea who the guy was.
Matthew has never been told that he has a different father than my uncle. Him and his sister, who's father is my uncle, look alike, so he's never wondered.
In my opinion, in this instance, it would be more damaging to tell him that his father is unknown. There is no way to find out who he is, and for me, the suspense would kill me, wondering for eternity.
In most cases, however, I agree that telling them from the beginning that they are adopted, so there are no surprises later. In addition, tell the siblings that the one child is adopted too. I know a woman from my former church who never told her other children that her 'first-born' wasn't her's. They were teens when they found out and were devastated. They felt robbed in some way.
I think adopting kids is giving them such a wonderful opportunity to have a life they probably wouldn't have had otherwise. It's a great thing.
__________________
I've gone and done it now! I'm on Facebook!!!