It does not take an expert to observe the many magnificent changes that take place in a human being from the time of birth through early childhood and beyond. Parents lovingly mark these changes in baby books and with photographs. Other relatives remark at the new abilities that babies seem to acquire daily. While parents may have just one or a few children to observe, developmental psychologists study many more. By studying many children over time, experts can chart the changes, and then begin to explain how they occur.