Here's a ... uh ... children's book.

Well, I guess that about explains it... German style. (Via Metafilter.) I'm trying to think how that would have affected me when I was a kid. It's just so damned clear! Wait 'til they ask, hand them the book, and see if they have any questions. I'm sure it will be better than my own insanely botched explanation. Or my mother's nuttily over-simplified version, in its entirety: "You know how men and women are physically built."