An 8 year old girl goes to her dad who is working in the yard and asks,
"Daddy, what's sex?"

The father is surprised that she would ask such a question, but decides
that if she is old enough to ask the question, then she is old enough to
get a straight answer.

He tells her about the birds and the bees; the egg and the sperm; and
the male and female.  When he has finished explaining, the little girl
is looking at him with her mouth opened, so the father asks her, "why
did you ask this question?"

The little girl explains that "Mom told me to tell you that dinner would
be ready in just a couple of secs."