In a similar vein:
It seems to me that there are three kinds of people in this sort of situation:
1. The friendly / curious who may be adoptive parents themselves* or are considering it;
2. The insufferable busybodies, and;
3. The intentionally hurtful / impertinent
Those in the first category deserve a civil answer. The others... Well, I can't punch them in the mouth or sic the grandmothers on them (this would, I think, rise to the level of an atrocity that would get the attention of the World Court and the UN; no need for all of that). So, what to do? My daughter will learn from me how she should respond and, more importantly, how she ought to feel about being an adoptee.
(*) My wife and I often remark to each other how we've developed radar for children who don't look like their parents (or, at least, the adults they are with): "Do you think they adopted, too?" Yet, it's awkward to say anything as we don't want to be boorish and invade somebody's privacy.