How do you apply game theory to select who should be on your Christmas shopping list? Can you predict Her Majesty's Christmas Message? Will calculations show Santa is getting steadily thinner - shimmying up and down chimneys for a whole night - or fatter - as he tucks into a mince pie and a glass of sherry in billions of houses across the world? Full of diagrams, sketches and graphs, beautiful equations, Markov chains and matrices, Proof That Santa Exists brightens up the bleak midwinter with stockingfuls of mathematiccal marvels. And proves once and for all that maths isn't just for old men with white hair and beards who associate with elves. Maths has never been merrier.