Megan McArdle examines the failures of magics and economics in the Harry Potter books.

Yet in the Potter books, the costs and limits are too often arbitrary.

A patronus charm, for example, is awfully difficult–until Rowling wants a stirring scene in which Harry pulls together an intrepid band of students to Fight the Power, whereupon it becomes simple enough to be taught by an inexperienced fifteen year old. Rowling can only do this because it’s thoroughly unclear how magic power is acquired. It seems hard to credit academic labour, when spells are one or two words; and anyway, if that were the determinant, Hermione Granger would be a better wizard than Harry. But if it’s something akin to athletic skill, why is it taught at rows of desks? And why aren’t students worn out after practicing spells?

(Via Instapundit.)