I've been experimenting with this one for fun, but it's doing something I rather it didn't. If you copy the code and observe it, you'll notice that a 3D trail of ellipses follows your mouse and changes constantly in size/color. After a while, however, the size changing stops and it's just a solid tube trailing after the mouse. If you click, you can erase and start a clean slate, but the size changing is still stopped. How can I fix it so it will continue to "jitter" size like at the beginning for a while longer?
Here it is:
Code:

float speed2=1; 
float gravity=0.1; 
float x, y, mx, my,speed; 
 
void setup(){ 
  size(900,600); 
  background(0); 
  smooth(); 
} 
 
void draw(){ 
  mx = mx + (mouseX-mx)/200;  
  my += (mouseY-my)/200;  
 
 
 
  speed=speed+gravity; 
  x=x+ speed; 
  y=y+speed2; 
 
 
  if((x>height)|| (x<0)){ 
 
    speed=speed*-1; 
  } 
 
  if((y>width)|| (y<0)){ 
 
    speed2=speed2*-1; 
  } 
 
  stroke(0,0,0,100); 
  fill(x,y,255,100); 
  ellipse(mx+random(2),my+random(2),x/6,x/6); 
 
  if(mousePressed){ 
    background(0); 
  } 
} 

you can switch to HSB colorMode and use the whole color Range
http://processing.org/reference/colorMode_.html
and maybe use map, to make it change throughout the whole width and hight, not only the first 255px...