My favorite re my adopted kids, that also applies to big families is "where did they come from?" My response is always, "heaven"
My MIL used to always say of her 4 kids in 5 years, that she checked them out at the library.
My friend had a baby this Oct, and she always told people that she ate watermelon seeds over the summer. That always cracked me up!
I actually got the "do you know what caused it? *snicker snicker*" thing from an ex-customer this week, and replied "not pulling out fast enough" out of annoyance. Certainly NOT the most couth and classy response, but it totally cracked up my sister. I think it just annoys me when people try to be all sly and cutesy while referencing my sex life.