Present it matter of factly, as if it were completely normal and kind of cool. Of course, it all depends, too, on the child's grasp of reproduction, as well. My kids have a pretty good take on it all, so if I had to tell them something like that (and hadn't already just been telling them all along) I'd probably wait until sometime in a quiet but happy conversation about the family and say something like, "Hey, did you know that the dad that started you growing inside of me was actually someone else besides Of course, I think it's kind of sad that his biological dad hasn't been part of his life all along. How did that happen?