// Learning Processing
  // Daniel Shiffman
  // http://www.learningprocessing.com
  
  // Example 23-2: Simple particle system with ArrayList
  
  // A simple Particle class
  class Particle {
    
    float x;
    float y;
    float xspeed;
    float yspeed;
    
    Particle() {
      x = mouseX;
      y = mouseY;
      xspeed = random(-1,1);
      yspeed = random(-2,0);
    }
    
    void run() {
      x = x + xspeed;
      y = y + yspeed;
    }
    
    void gravity() {
      yspeed += 0.1;
    }
    
    void display() {
      stroke(0);
      fill(0,75);
      ellipse(x,y,10,10);
    }
  }