Our family was built like this, two older boys, the old fashioned, biological way and two younger boys the new fangled, open, transracial adoption way. As one laughing observer put it, "you've got one of every color", so pretty much every where we go, everyone knows that we're a little different. 
It's been almost 5 years since our first adoption, so you think by now I'd be used to all the unwanted attention, the stares and the questions. For the most part I have found that people are just curious, and I often try to take the time for a little adoption education on correct terminology, etc. Occasionally, though, there will be that one person that just needs to be hit in the face with a chair. Now, I usually don't hit people in the face with chairs, so I usually resort to some sort of snarky comment, or perhaps I act like I'm a lot dumber that I am, like I don't quite understand what they're really trying to say.
The other day I ended up having all the kids at the orthodontists with me for my oldest son's appointment. I try to avoid this, as the younger kids are pretty rambunctious, and don't do well during long periods of time in small rooms surrounded by sharp things. But it happened, and we made it through. On the way out the door, I stopped at the desk to schedule next months appointment and it began. 
Now, the lady behind the desk is not the sharpest knife in the box, if you know what I'm saying, and I was already a little on edge after trying to keep the little kiddos happy for an hour when she said to me, "Four boys, wow! Are they all yours?" "Yes, they are, yes, it's a handful", I said. "Are they all brothers?" she said. "Yep" (big smile from me, as I can see where this is going) "No, I mean, are they all, you know, original brothers?" she stumbles through with a big frown on her face. "Yes," I said, "they are are mine and they are all brothers." 
By this time the drug rep sitting in the chair to the side of me is all smiles. I catch her eye and can tell she's really enjoying the show. "Oooooooooohhhhhhhhhh, wow!" said the receptionist, her voice dripping with judgement. She seemed to be highly disapproving of what I think she thought was the mixed fathering I had provided for my sons and what it might say about my moral character. I finally gave in and asked her if she meant, were my children adopted and did they share any biological parents. She just looked at me and then said, "oh, I guess so, yeah."
What's the take away here? Think about what you really want to know before you ask such a private question. Then think about why you want to know it? Are you just being nosy, even if you have no ill intentions? Or do you want to learn more about adoption? Is my family the first mixed race family you've ever seen? What is it you really want to know and is it even an appropriate question to ask a complete stranger?
My kids are right here, they can hear what you're saying. Don't talk about them like they are some exotic dog in a pet store. I'd be glad to share my experiences with you, but perhaps not while I'm trying to wrestle 4 boys in the middle of the grocery store.
Having the chance to be a biological and an adoptive mom makes me one of the luckiest moms in the world. Two amazing experiences that have stretched me and changed me in every way possible. One of the biggest things these experiences has taught me is think before you speak.