I am currently reading the excellent autobiography Experience by Martin Amis, from whence came this joke (as told to the author by his father, Kingsley Amis, and before that by his father to him)

This is paraphrased, as I don't have the book to hand

Life is good on the farm, where little Davy is growing up in the sunshine and approaching his twelfth birthday.
One day in the kitchen Ma says to Pa;
"Young Davy's growing up fast, isn't it about time you took him to one side and had a little chat about The Birds and the Bees?"
"Yes Ma, I suppose you're right. There comes a time in every young man's life when it's only right that he knows a few things about nature. I'll 'ave a talk to him."

So, the next afternoon in the Orchard - a blissful summer's day, the butterflies fluttering, the bees buzzing, and the birds flying high in the clear blue sky - Pa and Davy are sitting under a tree contemplating nature.
Pa says to Davy,
"You know son, there's something it's time that you we told, something every growing boy needs to know".
"Yes Pa?"
"Davy, you know what we were doing with those two girls in the ditch last night? The Birds and the Bees do it too!"