There's an interesting, two-page article linked below. We've been asked all of these questions many, many times over, and many more that are far more invasive (like the fellow Walmart shopper whose third or fourth question was whether my children's "real" parents died of AIDS...and yes, the children were standing right there).
One question that I never understand is the almost-obsessive need that people (including perfect strangers) have to ask whether Seth and Lizzie are siblings. Matthew, in particular, is sensitive to how I answer that question. So I usually answer something like "yes, they are all three siblings." But that answer has never once been enough to satisfy people's curiosity - there's always the follow-up winner where the asker points at my two younger children and says "yes, but I mean those two?" Sigh. Really?
I get that we are a conspicuous family, but really, I don't feel a need to share our personal journey with a perfect stranger...and nor would I ask you about the means by which you produced your children or about why one of them looks different than the other, and so on and so on. Just tell me I have beautiful children (because I do, so it's easy to say) and that I must have my hands full (because, again, I do!)!
Anyway, before I climb onto my soapbox, here's an article that says it all in a gentler way that I possibly could here!
Eight Things Never to say to Adoptive Families