Keeley Hawes was asked by The Guardian's Saturday magazine to tell a joke. Someone well known (or who wants to be well known) gets asked this question most weeks.
This one was one of those rare occasions when I laughed out loud...
A little boy looks up at his dad and asks "Daddy, where does poo come from?"
The dad harrumphs a bit and says "Well, it's a bit complicated; when you eat your food, it all goes down into your tummy and then your tummy takes all of the goodness out of the food and the rest comes out as poo."
The little boy looks a bit confused but accepts the answer. After a moment's silence the little fellow asks "So, daddy, where does Tigger come from?"