She must have overheard this someplace else

Tonight at dinner:

Elizabeth: I'm going to be a big sister.
Me: Yes, yes you are.
E: 'Cause I have a baby in my tummy.
M: You do?
E: Yes. Oooh. The baby is really moving around.
M:
E: My tummy hurts because the baby is really kicking!
M:
E: Wow, feel that!
M: !!