float bx;
  float by;
  int bs = 20;
  boolean bover = false;
  boolean locked = false;
  float bdifx = 0.0; 
  float bdify = 0.0; 
  
  void setup() 
  {
    size(200, 200);
    bx = width/2.0;
    by = height/2.0;
    rectMode(CENTER_RADIUS);  
  }
  
  void draw() 
  { 
    background(0);
    
    // Test if the cursor is over the box 
    if (mouseX > bx-bs && mouseX < bx+bs && 
        mouseY > by-bs && mouseY < by+bs) {
      bover = true;  
      if(!locked) { 
        stroke(255); 
        fill(153);
      } 
    } else {
      stroke(153);
      fill(153);
      bover = false;
    }
    
    // Draw the box
    rect(bx, by, bs, bs);
  }
  
  void mousePressed() {
    if(bover) { 
      locked = true; 
      fill(255, 255, 255);
    } else {
      locked = false;
    }
    bdifx = mouseX-bx; 
    bdify = mouseY-by; 
  
  }
  
  void mouseDragged() {
    if(locked) {
      bx = mouseX-bdifx; 
      by = mouseY-bdify; 
    }
  }
  
  void mouseReleased() {
    locked = false;
  }