Based on her observations, Maria Montessori determined that children pass through different planes of development in three year cycles. In the primary, this includes the traditional kindergarten year and may be extended to a fourth year depending on a child's birth date. Whether in toddler, primary, lower elementary or upper elementary, first year children learn both from their teachers as well as from observing, imitating and emulating the older children. By the time children reach their third year in each cycle, they have developed a sense of responsibility and mastery and share their knowledge and skills with their younger classmates.