'Mummy, my poo comes from my willy doesn't it?'
'No darling, poo comes from your bottom. Wee comes from your willy.'
'Nooo Mummy, it comes from my willy! I want it to come from my willy.'
'Well it's not really up to Mummy, it's the way you're built. Everyone's poo comes from their bottom. Mummy doesn't even have a willy!'
(crying now) 'Nooooooooo MY poo comes from MY willy'
(noncommittal) 'OK darling'
Sometimes you have to know when to give up the argument.