My granddaughter was about 4-1/2 when her baby brother was born and she
had never seen a penis. Anyway, when she was little I would tell her
"let's go change your foo-foo" to change her diaper which later changed
to "foo."

Anyway, Alex was maybe 1 day old in the hospital and she was standing on
a chair leaning over the bed and my daughter was changing his diaper.
She got a really strange look on her face and said "Mom, how come the
baby doesn't have a foo?" After we stopped laughing we explained that
boy "foos" and girl "foos" were different and she was okay with that,
but still smile when I think about that one and the look on her face.