What is the fascination of paint by numbers? Is it the intoxicating and compulsive act of filling in small pools of color? Or the easy thrill of creating your own impressionist masterpiece? Or a fond nostalgic yearning for a craze that cut across national boundaries and age groups? Invented in 1951 by Dan Robbins-based on an idea used by Leonardo da Vinci to teach painting-the paint-by-number craze reached its zenith in the 1950s but continues even...