int radius = 20;
PVector previous = new PVector(0,0);

void setup() {
 size(512,512);
 frameRate(24);
 pos = new PVector(random(0,width),random(0,height));
 vel = new PVector(-1,10);
 for(int i = 0;i<number;i++) {
  prev[i]=pos;
 }
}


void draw() {
  background(255);
  if(pos.x>=width-radius||pos.x-radius<=0) {
    vel.x*=-1;
  }
  if(pos.y>=height-radius||pos.y-radius<=0) {
    vel.y*=-1;
  }
  prev[0]=pos;
  for(int i = 2;i<=14;i++) {
   prev[i]=prev[i-1];
   fill(200);
   ellipse(prev[i].x,prev[i].y,2*radius,2*radius);
  }
  fill(0);
  pos.add(vel);
  ellipse(pos.x,pos.y,2*radius,2*radius);
}