// Learning Processing
  // Daniel Shiffman
  // http://www.learningprocessing.com
  
  // Example 5-9: Simple Gravity
  
  float x = 100;   // x location of square
  float y = 0;     // y location of square
  
  float speed = 0;   // speed of square
  
  // A new variable, for gravity (i.e. acceleration).   
  // We use a relatively small number (0.1) because this accelerations accumulates over time, increasing the speed.   
  // Try changing this number to 2.0 and see what happens.
  float gravity = 0.1;  
  
  void setup() {
    size(200,200);
  
  }
  
  void draw() {
    background(255);
  
    // Display the square
    fill(175);
    stroke(0);
    rectMode(CENTER);
    rect(x,y,10,10);
    
    // Add speed to location.
    y = y + speed;
    
    // Add gravity to speed.
    speed = speed + gravity;
    
    // If square reaches the bottom
    // Reverse speed
    if (y > height) {
      // Multiplying by -0.95 instead of -1 slows the square down each time it bounces (by decreasing speed).  
      // This is known as a "dampening" effect and is a more realistic simulation of the real world (without it, a ball would bounce forever).
      speed = speed * -0.95;  
    }
  }