We called him the Professor. And he called my son Root, because he said, the flat top of his head reminded him of the square root sign. "There's a fine brain in there," the Professor said, mussing my son's hair. Root, who wore a cap to avoid being teased by his friends, gave a wary shurg. "With this one little sign, we can come to know an infinite range of numbers, even those we can't see." He traced the symbol in the thick layer of dust on his desk. Of all the countless things my son and I learned from the Professor, the meaning of the square root was among the most important.