When my son was about 5, he asked me where poo came from.

I gave him a short concise answer on how you eat, and the food goes down into your tummy where the good stuff gets taken out of it to fuel your body, then what's left passes down to a long tube called intestines where it is mixed with nasty stuff, all which comes out of your bum when you go to the toilet.
He looked at me in horror and said "What about tigger?"