     private let playerNode: PlayerNode = PlayerNode()
     private let enemies =  SKNode()
     private let playerBullets = SKNode()
+    private let forceFields = SKNode()
     
     class func scene(size:CGSize, levelNumber:UInt) -> GameScene {
         return GameScene(size: size, levelNumber: levelNumber)

        
        // Player bullets
        addChild(playerBullets)
        
        // Physics body delegate
        physicsWorld.gravity = CGVectorMake(0, -1)

        view!.presentScene(gameOver, transition: transition)
    }
    
    private func createForceFields() {
        let fieldCount = 3
        let size = frame.size
        let sectionWidth = Int(size.width)/fieldCount
        for var i = 0; i < fieldCount; i++ {
            let x = CGFloat(i * sectionWidth +
                arc4random_uniform(UInt32(sectionWidth)))
            let y = CGFloat(arc4random_uniform(UInt32(size.height * 0.25))
                + UInt32(size.height * 0.25))
            
            let gravityField = SKFieldNode.radialGravityField()
            gravityField.position = CGPointMake(x, y)
            gravityField.categoryBitMask = GravityFieldCategory
            gravityField.strength = 4
            gravityField.falloff = 2
            gravityField.region = SKRegion(size: CGSizeMake(size.width * 0.3,
                size.height * 0.1))
            forceFields.addChild(gravityField)
            
            let fieldLocationNode = SKLabelNode(fontNamed: "Courier")
            fieldLocationNode.fontSize = 16
            fieldLocationNode.fontColor = SKColor.redColor()
            fieldLocationNode.name = "GravityField"
            fieldLocationNode.text = "*"
            fieldLocationNode.position = CGPointMake(x, y)
            forceFields.addChild(fieldLocationNode)
        }
    }
    
    
}
