I remember learning all kinds of tricks when I was young, but not how they worked - possibly because the people who showed them to me didn't know either. Teaching children new skills and tricks certainly stimulates them, but if you go no further you are doing them a disfavour. Inquisitiveness is natural to a child, but it has become an essential weapon in the armoury of an adult. One of the aims of this book is to encourage children to dig deeper every time they learn something new.Number Magic is the second book I have written for my elder granddaughter, a lively 8-year-old who loves everything, from mathematics and painting to ski-ing and playing the piano. An avid reader, she was over the moon a few months ago to find that I'd written a real Kindle book - Secret Writing - specially for her, but told me she wanted to finish the second Harry Potter book before starting on it. I am sure the world is full of boys and girls like her, whose parents are always looking for ways to challenge and amuse them, so I offer the Grandpa Mike KH series as challenging books for those who are still too young to tackle Martin Gardner or Simon Singh.Young children gain knowledge and skills so rapidly that they quickly outgrow a lot of their books. I have deliberately structured Number Magic so that the young reader can skip the difficult bits, trusting that their innate curiosity will make them return to tackle them later. This also means that the book has enough real meat in it to appeal to quite a few adults. For instance, can you easily add two long numbers in your head? Or do you understand number-based card tricks well enough to modify them?