Indeed, "'Aha!'" she cried etc." sounds like a music hall comedian's catch-phrase -- "If my aunt had wheels she'd be a bicycle" -- or a line from a popular recitation -- "per tuppence per person per trip!" My Dad used to drive small kids nuts with the rhetorical question, "Why is an orange?" to which the correct response was, "Because a duck has no armpits."

It's a British thing, and part of the culture that gave rise to the Goons and Monty Python.