"Little by little, the child develops," wrote an undergraduate in a friend's cognitive development class, and so, for the most part, it is. But what explains the U's of cognitive development? Namy, Campbell, and Tomasello and Cashon and Cohen take a standard approach to understanding U-shaped curves: as the product of a mix of different cognitive processes that come on-line at different points in development. Gershkoff-Stowe and Thelen take a rather different view, suggesting that one should look outside the head, away from "programs" and "modules" and out toward the body and the environment. The none-too-surprising conclusion is that every U-shaped phenomenon must be studied independently. Phenomena that have the appearance of U-shaped development should never be taken at face value--the base rates are too low. In most behaviors, performance really does steadily improve, and some seemingly U-shaped phenomena may reflect little more than measurement error. (Contains 1 footnote.)