Want to impress an audience? Then why not become a lightning calculator by learning Burkard Polster and Marty Ross' method for working out the day on which someone was born from their birthday really fast.
Maths for mums and dads by Rob Eastaway and Mike Askew. This book is an absolute triumph. Given the authors' reputations, I would expect nothing less, so it is something of a relief to be able to write that first sentence.