From the early childhood game of Peek-a-Boo to an unexpected bouquet of roses after a hard day at work, the pay-off, neuroscientists tell us, is a rush of dopamine, intensifying our emotions by as much as four hundred percent. How do we use this human affinity for surprise in our mystery fiction? No mystery there. Overturning the expectations of readers, surprising them, is called a “plot twist,” that moment when everything the reader has been led to believe turns out to be wrong.