The one about the baby-bringing birds...

Little Johnny says "Mum, what kind of bird brings white babies?"

His mum says, "Why, a stork, little Johnny."

Little Johnny says, "Mum, what kind of bird brings black babies?"

His mum says, "A raven, dear."

Little Johnny then says, "Then what kind of bird brings no babies at all?"

His mum says, "A swallow!"