In 1752 Voltaire published Micromegas, the story of a 120,000-foot tall resident of a planet of Sirius who visited our solar system. As a parting gift, the visitor gave the French Academy of Sciences a book that, he said, contained the answer to all things. On examination, the book was found to be blank. This is the riddle: why was it blank? Voltaire's Riddle contains a new translation of the story and continues with a series of chapters, each of which begins with a historical or literary vignette followed by the mathematics behind it. Topics include trajectories of comets, the flattening of the Earth at the poles, Maupertius's pursuit problem, Durer's possible use of trochoids, and the precession of the equinoxes. The book ends with possible answers to Voltaire's riddle. Readers need know little more than calculus.