Below you will find an applet that animates what is is happening in this example. The red line segments represent the gutter. The yellow rectangle represents the cross sectional area. Drag the green point to change the dimensions of the gutter. As you do this, to the left, you will see a blue point tracing out the graph of the cross sectional area as a function of the value of x (or the height). You can start the trace over by clicking on the red x in the lower right-hand corner of the applet.