Piaget's theory of cognitive development provides tools to help avoid problems in mathematics learning. Specifically, the concept of equilibration, where new experiences are either assimilated into existing understanding or schemes are modified to accommodate new experiences. A teacher who is mindful of this process can better guide lessons to result in sophisticated understanding and robust schemes. Piaget's theory outlines four stages of cognitive development: sensori-motor, pre-operational, concrete-operational, and formal operational.