Arithmetic Mouse, A. Mouse for short, tells the story of his famous ancestor, Moray Mouse, who could not use her Metamorphic Mouse powers because she did not know the multiplication facts. Moray just happen to live in the Merchiston Castle that belonged to the famous mathematician John Napier in the early 1600s. Read how John Napier's own struggles and subsequent invention helped Moray to learn the multiplication facts through brain friendly learning patterns.