Mama, what's a "wife"?

Miles and I were singing "Farmer in the Dell" the other night, when he asked me, "Mama, what's a wife?" ("The farmer takes a wife...") Hm, how to answer?

Grace: A wife is a woman who's married to someone. Grandma Kathy is Grandpa Chuck's wife, Ah-Ma is Ah-Gong's wife, and I'm sort of Daddy's wife...
Miles: But *I* want a wife!
Grace: Well, maybe you'll have one when you're older.
Miles: But I want one now!
Grace: You'll have to wait 'til you're a grown-up.
Miles: The robot is my wife!!! (Referring to toy robot on windowsill.)