// Generates the pipes in FlappySwift.
    func generatePipes() -> SKNode {
        // The node that represents all of the pipes.
        let pipes = SKNode()
        
        // Load the textures.
        let pipeTextureBottom = SKTexture(imageNamed: "PipeUp")
        let pipeTextureTop = SKTexture(imageNamed: "PipeDown")
        pipeTextureBottom.filteringMode = .Nearest
        pipeTextureTop.filteringMode = .Nearest
        
        // Animation that moves the pipes from right to left.
        let distanceToMove = CGFloat(
            self.frame.size.width + 2.0 * pipeTextureBottom.size().width)
        let movePipes = SKAction.moveByX(
            -distanceToMove, y:0.0,
            duration: NSTimeInterval(0.01 * distanceToMove))
        
        // Animation the removes pipes.
        let removePipes = SKAction.removeFromParent()
        
        // Put them together.
        let movePipesAndRemove = SKAction.sequence([movePipes, removePipes])
        
        // Animation which spawns the pipes at a random height.
        // But always makes the pipes 150 pixels away from each other.
        let spawn = SKAction.runBlock({ () -> Void in
            let pipePair = SKNode()
            pipePair.position = CGPointMake(
                self.frame.size.width + pipeTextureBottom.size().width * 2,
                0)
            let maxHeight = UInt32(self.frame.size.height / 6)
            let bottomHeight = arc4random() % maxHeight + maxHeight
            
            // Set up the bottom pipe.
            let bottomPipe = SKSpriteNode(texture: pipeTextureBottom)
            bottomPipe.setScale(2.5)
            bottomPipe.position = CGPointMake(0.0, CGFloat(bottomHeight))
            bottomPipe.physicsBody = SKPhysicsBody(
                rectangleOfSize: bottomPipe.size)
            bottomPipe.physicsBody?.dynamic = false
            
            // Set up the top pipe.
            let topPipe = SKSpriteNode(texture: pipeTextureTop)
            topPipe.setScale(2.5)
            topPipe.position = CGPointMake(
                0.0, CGFloat(bottomHeight) + topPipe.size.height + 150.0)
            topPipe.physicsBody = SKPhysicsBody(rectangleOfSize: topPipe.size)
            topPipe.physicsBody?.dynamic = false
            
            // Add them together into the pipe pair.
            pipePair.addChild(bottomPipe)
            pipePair.addChild(topPipe)
            
            // Animate the pair.
            pipePair.runAction(movePipesAndRemove)
            
            // Add to our pipes.
            pipes.addChild(pipePair)
        })
        
        // Pause.
        let delay = SKAction.waitForDuration(NSTimeInterval(2.0))
        
        // Put the spawn and the delay together.
        let spawnThenDelay = SKAction.sequence([spawn, delay])
        self.runAction(SKAction.repeatActionForever(spawnThenDelay))
        
        return pipes
    }