I haven't come across that one yet. My thing is because my DS looks like me, people assume I was pregnant when I wasn't:
Stranger: You're too thin to have had a baby!
Me: Just lucky, I guess.
Stranger: Is your husband big too?
Me: I'm not sure where he gets it from.
I guess after a while you get really good at coming up with the quick non-answers.
[On a side note: When it's people that we see all the time who are relative strangers being nosy (such as the people who work at the grocery store where I shop), I really don't want his adoption to be his entire identity so I don't bring it up. Adoption is only a small part of who he is! He's only 3.5 months old now. I'll let him be the one to tell people he was adopted (when he's old enough) if he so chooses.]
It'd be great to get a catalogue of goofy comments made by strangers and snappy comebacks from parents about adoption. It'd be quite helpful for those of us just starting out! We've all been there, right?