You can reach any point just moving front or back, up or down, and side to side. For example, if you saw a bunch of kids in a playground, you might estimate that there were fifteen children playing. You wouldn't guess 500 children, unless it was a huge playground, and you wouldn't guess 0 children, because you know there are some kids there. Whenever you estimate, you should check to see that your answer makes sense. In math, a line is always straight.
Lines go on forever in both directions.
Here is a picture of a line. A real line is only one- dimensional , but the picture is bigger, so you can see it more easily. Negative A negative number is a number less than 0. For example, -10 means negative ten. You can imagine this as a temperature ten degrees below zero.