Explain what a number line is and make a number line by making a physical number line (with number cards and a clothes line) or by drawing one on the interactive whiteboard. Tell students that the lowest number always is at the start, so the left side, that every line represents one number, and that the numbers are in the same order as the counting sequence. Discuss with students that you always count forwards from left to right, and that you always count backwards by counting right to left. Point at the numbers as you do so, and then have the students count out loud with you.
Check that students understand number lines and numbers to 20 on the number line by asking the following questions:
What is a number line?
How do you put a number on the number line?
Where do you start counting?
When do you count forward and when do you count back?