This comes from a friend, who claims it happened to her.

Her seven-year-old asks: Mom, whats sex?

Mom (flustered): Well, thats whether youre a girl or a boy. You know,
like when we signed you up for swimming lessons, and the form asked what
your sex was, thats what they wanted to know.

Son: OK.

Mom: (whew)

Next day:

Son: Mom, Joe says that sex is when you take off your clothes and rub
against each other. Is that true?

Mom: Well, yes, thats another meaning of sex.

Son: ALL your clothes?

Mom: Yes.

Son: YUCK! Really?

(Ive forgotten what transpired here; no matter.)

Mom: Well, Im real glad you asked me about this. If you have any more
questions, please ask.

Son: Well, I do have one more.

Mom (breaking into a sweat): Yes?

Son: Where does metal come from?