Clicking numbers adds them to a String in the World. Later, they are turned into doubles by parseDouble(), which might cause the problem. I will look into that. The graphing calculator sounds like a cool idea, and can be a nice extension to this.

Hmm maybe you should make it impossible for the ball to go straight ahead when SPACE is pressed... it bounces back and forth until it reaches the center.

Wow, I notice that there is a sudden lag in the game once the cursor enters the frame. Must be because the picture change for the cursor makes the game lag .