     
     let gravity = UIGravityBehavior()
     
+    lazy var collider: UICollisionBehavior = {
+        let lazilyCreatedCollider = UICollisionBehavior()
+        lazilyCreatedCollider.translatesReferenceBoundsIntoBoundary = true
+        return lazilyCreatedCollider
+    }()
+    
     lazy var animator: UIDynamicAnimator = {
         let lazilyCreatedDynamicAnimator = UIDynamicAnimator(referenceView: self.gameView)
         return lazilyCreatedDynamicAnimator

    override func viewDidLoad() {
        super.viewDidLoad()
        animator.addBehavior(gravity)
    }
    
    var dropSize: CGSize {

        gameView.addSubview(dropView)
        
        gravity.addItem(dropView)
        collider.addItem(dropView)
    }
}

