I was born on January 31, 1979—a Wednesday. I know it was a Wednesday, because the date is blue in my mind and Wednesdays are always blue, like the number 9 or the sound of loud voices arguing. I like my birth date, because of the way I'm able to visualize most of the numbers in it as smooth and round shapes, similar to pebbles on a beach. That's because they are prime numbers: 31, 19, 197, 97, 79, and 1979 are all divisible only by themselves and 1. I can recognize every prime up to 9,973 by...