         self.physicsWorld.contactDelegate = self
         self.physicsBody = SKPhysicsBody(edgeLoopFromRect: self.frame)
         self.physicsWorld.gravity = CGVectorMake(0.0, -5.0)
+        self.physicsBody?.categoryBitMask = CATEGORY_ROUND
+        self.physicsBody?.contactTestBitMask = CATEGORY_BALL
         
         drawBackground()
     }

    
    let CATEGORY_RACKET: UInt32 = 0x00000001
    let CATEGORY_BALL: UInt32 = 0x00000010
    
    // not supported construct.
    required init(coder aDecoder: NSCoder) {

        ball.physicsBody = SKPhysicsBody(circleOfRadius: width)
        ball.physicsBody?.dynamic = true
        ball.physicsBody?.categoryBitMask = CATEGORY_BALL
        ball.physicsBody?.contactTestBitMask = CATEGORY_RACKET | CATEGORY_ROUND
        self.addChild(ball)
    }
    
