True.
Teaching something to someone else requires a deep understanding of the material. In order to effectively teach a concept, one must be able to explain it in a clear and concise manner, answer questions that may arise, and provide examples to help clarify the concept. This process requires a level of mastery of the material, as well as the ability to communicate it effectively. When teaching something to someone else, the teacher is also reinforcing their own knowledge and solidifying their understanding of the material. Therefore, teaching can be an effective way to learn and improve one's own mastery of a subject.