How do you add three-digit numbers without even a pencil and paper? Can a mirror be used to measure the height of a building? And if dinner cost you $37.53, how much should the tip be? Whether you want to guesstimate the total for the items in your shopping cart, convert Celsius to Fahrenheit, remember Roman numerals, or use your watch as a compass, there are surprisingly simple strategies at your fingertips. Along with techniques and mnemonic devices that you can use in a casino or your math or algebra class, here too are the secrets of calculating your body-mass index, yearly salary, and—with the help of crickets—the temperature.