In geometry, a straight line is a collection of one or more points, which can be regarded as extending infinitely in the same direction. Because the long side of the blackboard is straight, it can extend infinitely in one direction, which conforms to the definition of straight line.

The characteristics of a straight line include: it is the shortest distance between two points, it can extend indefinitely, it can bend but it is still a straight line, and so on. These features can also be applied to the long sides of the blackboard.

When we consider the use of the blackboard, we can also find its relationship with the straight line. Blackboards are usually used to show and explain mathematical knowledge, figures and words, which can be represented by straight lines. For example, equations and figures in mathematics can be represented by straight lines, and words can also be arranged by straight lines to form a meaningful sentence or paragraph.

The purpose of the blackboard:

1. Writing: The most basic and common use of the blackboard is writing. Teachers can write down important concepts, formulas or sentences on the blackboard so that students can better understand the course content. Teachers can also guide students to think and stimulate students' interest and curiosity by writing on the blackboard.

2. Demonstration: The blackboard can be used to present various graphics, images and videos to better explain and explain the course content. For example, a math teacher can use the blackboard to show mathematical figures and equations, a history teacher can use the blackboard to show pictures and words of historical events and people, and a science teacher can use the blackboard to show the steps and results of scientific experiments.

3. Interaction: The blackboard is also a tool to promote the interaction between teachers and students. Teachers can guide students to actively participate in classroom activities by asking questions on the blackboard or discussing with students. Students can also show their ideas and creativity by writing or drawing on the blackboard. This kind of interaction can stimulate students' interest and motivation in learning and improve their thinking ability and creativity.

4. Used for recording and recalling: The blackboard can be used to record important contents in class and students' ideas, which is convenient for students to review and review. Blackboard can also be used as a memory tool to help students recall the key points and difficulties in class, which is helpful to improve students' learning effect.