My mother tried to explain it when I was nine, by starting with amoebae and working her way up the evolutionary ladder. She was a shy person, however and having gotten as far as frogs and fishes spawning, gave up. I'd have been terrified of swimming in the same lake as a boy, but fortunately, we were farm kids, and I knew what bulls and cows got up to - looked pretty owie to me, but at least didn't involve clouds of milt.

I'd also witnessed artificial insemination of a cow, which totally grossed out my five year old self!