Zahlvergnugen: Everything you always wanted to know about math but didn't know how to ask

This title covers math from arithmetic shortcuts to calculus and beyond, all on a level (it is hoped) accessible to the average adult. It is designed for the person who now knows little beyond 9 x 9 = 81, but who would like to learn more; and the person who did study some "advanced" math in high school or college, but now has forgotten all but the formula for the area of a circle.