How did Piaget develop most of his theories about children's development?

Piaget developed most of his theories about children's development primarily through observing his own children. This hands-on approach allowed him to witness firsthand the cognitive processes as they unfolded in real time. By carefully analyzing their behaviors, interactions, and problem-solving methods, he formulated his stages of cognitive development, including the sensorimotor, preoperational, concrete operational, and formal operational stages.

His observations provided rich, qualitative data that informed his understanding of how children think, learn, and grow. This emphasis on developmental stages was revolutionary and highlighted the importance of active exploration and interaction with the environment in a child's learning process. Through this extensive observational work, Piaget was able to outline distinct patterns in children’s thinking and reasoning, which laid the groundwork for future research in child development.