"What is perimeter" is the first lesson of Unit 5 "perimeter" in the first volume of the third grade of primary school mathematics published by Beijing Normal University. The main teaching content of this section is to understand the meaning of perimeter through practical activities such as drawing, measuring and touching, so as to pave the way for understanding the meaning of perimeter of various graphics and calculating perimeter in the future. Plan to teach a class.

The textbook of this lesson has the following characteristics: first, students can initially and intuitively understand the circumference of leaves by ants crawling around the edge of leaves; Then understand the meaning of perimeter through operation activities; Finally, through practical activities, let students further experience the close relationship between circumference and real life.

The method of "conversion" is an effective thinking method to study and solve mathematical problems. It is a way of thinking that uses the viewpoint of movement, change and development of things and the relationship between things to turn the unknown into the known and the complex into the simple. After-class exercises permeated the students with the mathematical thought of translation. Therefore, in teaching, we should try our best to combine the specific teaching content, infiltrate the idea of mathematical reduction, and consciously cultivate students to learn to solve problems with the idea of mathematical reduction, thus improving students' mathematical ability.