umm.... what? And I think they are yellow by the way so I dont want to prove them green Ok let me explain. I'm assuming you know how to play tennis. When you hit the ball and you suck, you just sorta smack it, however when the pro's hit it you here this "thump". With a low-compression tennis ball, almost any amuteur tennis player can make that "thump" sound, which means its much easier to play tennis. (don't ask, whenever you hear the "thump" that means that was a good hit)