The author advocates for writing as an essential communication skill for learning mathematics. Mathematical diary writing is cited as a good way for students to privately represent their thinking through pictures, language, or symbols, and also as a channel for children to communicate with themselves and with their teachers. Cited research demonstrates that diary writing also has benefits for teaching and learning by nurturing number sense, helping teachers investigate children's understanding and feelings about lessons, and giving children a way to formulate their thinking. The author contends that children need to use their own ways to explain what they learn in class, and that mathematical diary writing promotes children's problem solving ability through explaining their ideas. (Contains 3 figures.)