"Why does my shit stink?" he asked politely.
"Don't swear." she said, "The word to use is faeces."
"Oh" he hesitated "Or poo perhaps?"
"Too childish, dear, although appropriate.
You're old enough to use more adult words.
But it's a sensible question nonetheless.
I presume it's all to do with your evolution.
It smells so bad so that you don't ingest it.
So that you're very careful to avoid it."
"What does 'ingest' mean?" "Well, eat,
or somehow get it in your mouth." she said.
"I see. So people whose shit didn't stink -
sorry, faeces, would sometimes ingest it
and get ill and die and fail to breed."
"Yes, something like that. Perhaps." she added.
"And now get back to that evolution homework
that you're avoiding, by going to the toilet."
"And we're evolved from all the stinky people
and all the nice-smelling ones just went extinct."
"I hope you washed your hands. Goodbye."