The Elf on the Shelf has been given their own mystery to solve. On Christmas Eve, a letter arrives at the North Pole from a little girl called Sofi. She has one question - how old is Santa Claus? And so begins a quest for two little Elves to find out the answer! But will Santa reveal how old he is? From the global phenomenon, The Elf on the Shelf, comes this gorgeous picture book which taps into the magic and wonder of Christmas.