Marriage rules

Conversation on the way into swimming today:

N: mommy, will you still be my mom when I'm a grown-up?
M: of course!
N: *thinks* Can I marry you when I grow up?
M: nope
N: *thinks*
M: honey, I'll always be your mom, even when you're a grown-up. Just like how Grammy is still my mom and I'm a grown up.
N: *thinks*. Can I marry Grammy when I'm a grown-up?
M: nope. If you decide to get married you have to pick someone who's not in your family.
N: why?
M: that's just the way getting married works
N: did God make it that way?
M: um, excellent question.

And then I said a silent prayer of thanks that we reached the changing room and he got distracted.