A little girl runs out to the yard where her father is working, and asks him, "Daddy, what's sex?" Her startled father sits her down, and tells her all about the birds and the bees.  He goes on to tell her about puberty, menstruation, erections, wet-dreams... and, he thinks -- what the hell -- and goes on to tell her the works.  He covers a wide assortment of topics and by the time he's finished, his daughter is somewhat awestruck with this sudden influx of bizarre new knowledge.
Her father finally asks: "So what did you want to know about sex for?"
"Oh, mommy said to tell you lunch would be ready in a couple of secs..."