Drawing a Line

If you know that two points are on a line, you can find the rest of the line. Let's watch Joan draw a line through these points:

(We haven't drawn the x-axis or the y-axis on this picture, because we don't need them to keep track of where we are.)

Joan shifts her tongue around so she can reach the two points, then shoots it straight through them:

The straight part of Joan's tongue is so sticky that it leaves a green mark behind.

Joan draws arrows on each end of her line, to show that it really goes on forever.

Your tongue probably isn't straight enough to draw lines with. Instead, you should use a ruler or something else with a long, straight edge.