There are plenty of books out there that lead the reader through numerical gymnastics. If you want to know how to multiply 45 times 25 in your head, this isn't the book for you. (Hint: 25 is 100 divided by 4.; Hint 2: 45x25 = (40 + 5)(20 + 5)) Our goal is a different, and I believe, a much deeper one. This book is mostly about deriving things without paper and pencil, which is how the title arose. We'll tackle some fairly easy derivations, like why...