Explain the number line to students. A number line is a line with numbers going up and down infinitely. It can have both negative and positive numbers on the line, though in the second grade students will usually work with positive numbers.
Show a number line to the class. This can be a line with numbers marked in even increments, or one with a picture above the line like a train to better illustrate the line. The train illustrates the number line by each train car breaking the line up into equal increments, giving students a visual to remember the number line.
Show the class how the number line is used. In the second grade, number lines are used to help illustrate math skills like adding and subtracting. The lines usually go up in increments of 1, and the students can count forward or backward to find the math answers. For example, a number line might have numbers 1 through 10 labeled and then have math problems below it like 1 + 4. The students start at the 1 and count up four numbers to get the answer 5.
Provide the class with number line worksheets. The worksheets will have math problems like addition and subtraction on them with a number line at the top. The students will understand best when they have some practice using the number line.