As Montessorians come to the beginning of the second century of the Montessori system of education, they are in agreement that following the principles and guidelines set out by Maria Montessori a century ago empowers them to present to children an environment that truly "works." As the child is very different and the changes very evident between one stage and the next, it is important to investigate why children pass through these physical and psychological changes during their growth stages. This article presumes to answer these following questions: (1) What are the reasons for these stages?; (2) Why isn't it better for the children to just grow smoothly from birth to maturity with no abrupt changes?; (3) Why are they programmed to have such different stages?; and (4) Why are we able to say that Montessori has been successful for 100 years? The author also describes the historical reasons for Montessori's planes of development and her educational system.