This comes from a friend, who claims it happened to her.
Her seven-year-old asks: "Mom, what's sex?"
Mom (flustered): "Well, that's whether you're a girl or a boy. You know, like when we signed you up for swimming lessons, and the form asked what your sex was, that's what they wanted to know."
Son: "Mom, Joe says that sex is when you take off your clothes and rub against each other. Is that true?"
Mom: "Well, yes, that's another meaning of `sex.'"
Son: "ALL your clothes?"
Son: "YUCK! Really?"
(I've forgotten what transpired here; no matter.)
Mom: "Well, I'm 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?"