What coordinate geometry principle is dynamically being illustrated in the applet below? [br](Feel free to move the BIG WHITE POINTS anywhere you'd like!)
[b]The [i]y[/i]-coordinate of the[/b] [color=#bf9000][b]midpoint[/b][/color] [b]of any segment drawn in the coordinate plane[/b] is equal to the arithmetic mean (average) of [b][color=#9900ff]the [i]y[/i]-coordinates of the segment's endpoints.[/color][/b] [br][br](Recall: The average of any two numbers is the same as the number that's located exactly halfway between them.)