One feature of functions is that as you move from left to right through the function's domain that the value of the function's range either remains [color=#1551b5]constant (blue)[/color], [color=#0a971e]increases (green)[/color], or [color=#c51414]decrease (red)[/color].
This applet can be used by teachers in a demonstration mode in the classroom or teachers can have students load it on their own computers as a worksheet to be completed for a grade. Left can change the left side of the blue box (most convenient size set).