After getting interested in prime numbers at a late stage of life, I factored all our friends' phone numbers, and inserted "math puzzles" in this year's holiday letters for them to solve. Very few solved them correctly to discover their phone number hidden among the problems. Most, in holiday haste, completely overlooked the puzzle. Quite a few refused to try them.

Pretty discouraged by then, I picked up The Children Star by Joan Slonczewski, and discovered 'jum , "Pig urine", a gifted but very poor young orphan girl. She loved to count things - light ships, sunbeams, bits of metal, digits of numbers to see if they were divisible by three, windows in office towers. I can really identify with that!

She was only about 5, but she created "families" of factors in her head. 693 is a family of four: a 7, 11 and twin 3's. "Huh?" you ask. 3 x 3 x 7 x 11 = 693. She thought of 693 as a family with four children. There was only one person in her family - her. Prime numbers were like her - orphans.

'jum also liked triangles. When someone asked if she could add, she said "Yes, three plus four is five." On another occasion she said one plus one is "a little less than one and a half." She could just see those right triangles in her head, and knew how long the sides were. With a couple of months playing in the holostage, she learned many more. When later asked to name two numbers squared that add to a third square, she said "152 and 1122. And 512 and 1402."

Adopted by a scientist who called her "Ushum", she enjoyed helping her new mother in her work. Together they helped their people understand a tiny but intelligent species that inhabited the Children Star, home to many other orphans. Ushum told her tiny friends to call her "187" because she was very fond of its prime factors, 11 and 17.

Well, she was too young to star in the whole book, so I had to accept villains and love themes, too. But there was more than enough science (including organic chemistry!) for any hard science fiction fan. And it's always fun to find an error. A prime chart with the aliens' codes shows the same code for two different numbers. (I corrected the error in my copy.) Joan Slonczewski is a biology teacher at Kenyon College in Ohio. Her writing held my interest and made me want to read her other books.