Please read this blog on adoption. We haven't adopted yet but I've already been fielding questions like this.
Person: I didn't know you were adopting.
Me: Yeah. We hope to be able to bring another child into our home sometime next year.
Person: Now, is Garrett yours?
Me: He's our biological son, yes.
In my head, "No. He isn't mine. We rent him from the neighbors when we go to church or on family outings. Of course he's mine. Just as an adopted child will be."