var Obstacle = Container.expand(function () {
	var self = Container.call(this);
	var obstacleGraphics = self.createAsset('obstacle', 'Obstacle', .5, .5);
	self.baseSpeed = -5;
	self.speedIncrease = 0;
	self.move = function () {
		self.x += self.baseSpeed - self.speedIncrease;
	};
});
var HeroBullet = Container.expand(function () {
	var self = Container.call(this);
	var bulletGraphics = self.createAsset('bullet', 'Hero Bullet', .5, .5);
	self.speed = 5;
	self.move = function () {
		self.y += self.speed;
	};
});
var Hero = Container.expand(function () {
	var self = Container.call(this);
	var heroGraphics = self.createAsset('hero', 'Hero character', .5, .5);
	var wingGraphics = self.createAsset('wing', 'Hero Wing', .7, .8);
	wingGraphics.direction = 1;
	self.speed = 0;
	self.bullets = [];
	self.gravity = 0.3;
	self.jump = function () {
		self.speed = -10;
	};
	self.update = function () {
		self.speed += self.gravity;
		self.y += self.speed;
		self.rotation = self.speed * 0.05;
		wingGraphics.y += wingGraphics.direction * 2 || -2;
		if (wingGraphics.y > 10 || wingGraphics.y < -10) {
			wingGraphics.direction = -wingGraphics.direction;
		}
		for (var i = self.bullets.length - 1; i >= 0; i--) {
			self.bullets[i].move();
			if (self.bullets[i].y > 2732) {
				self.bullets[i].destroy();
				self.bullets.splice(i, 1);
			}
		}
	};
});
var Enemy = Container.expand(function () {
	var self = Container.call(this);
	var enemyGraphics = self.createAsset('enemy', 'Enemy', .5, .5);
	self.speedX = -0.6;
	self.speedY = 0;
	self.verticalMovementRange = 50;
	self.verticalSpeed = self.verticalMovementRange / (5 * 60);
	self.verticalDirection = 1;
	self.initialY = 0;
	self.move = function () {
		self.x += self.speedX;
		if (self.initialY === 0) self.initialY = self.y;
		self.y += self.verticalSpeed * self.verticalDirection;
		if (Math.abs(self.y - self.initialY) >= self.verticalMovementRange) {
			self.verticalDirection *= -1;
		}
	};
});
var Game = Container.expand(function () {
	var self = Container.call(this);
	LK.stageContainer.setBackgroundColor(0x87CEEB);
	var hero = self.addChild(new Hero());
	hero.x = 2048 / 2 - 200;
	hero.y = 2732 / 2;
	var obstacles = [];
	var score = 0;
	var scoreTxt = new Text2('0', {
		size: 150,
		fill: "#ffffff"
	});
	LK.gui.topCenter.addChild(scoreTxt);
	scoreTxt.anchor.set(.5, 0);
	var isGameOver = false;
	var tickOffset = 0;
	var speedIncreaseFactor = 0;
	LK.on('tick', function () {
		hero.update();
		if (hero.y > 2732 || hero.y < 0) {
			isGameOver = true;
		}
		for (var i = obstacles.length - 1; i >= 0; i--) {
			obstacles[i].move();
			if (obstacles[i].x < -50) {
				obstacles[i].destroy();
				obstacles.splice(i, 1);
				scoreTxt.setText(score);
			}
			if (hero.intersects(obstacles[i])) {
				isGameOver = true;
			} else {
				for (var j = hero.bullets.length - 1; j >= 0; j--) {
					if (hero.bullets[j].intersects(obstacles[i])) {
						hero.bullets[j].destroy();
						hero.bullets.splice(j, 1);
						if (obstacles[i] instanceof Enemy) {
							score++;
							scoreTxt.setText(score);
							obstacles[i].destroy();
							obstacles.splice(i, 1);
						}
					}
				}
			}
		}
		if (tickOffset++ % 90 == 0) {
			var newObstacle = new Obstacle();
			newObstacle.x = 2048;
			newObstacle.y = Math.random() * 2732;
			newObstacle.speedIncrease = speedIncreaseFactor;
			obstacles.push(newObstacle);
			self.addChild(newObstacle);
		}
		if (tickOffset % 600 == 0) {
			speedIncreaseFactor += 0.5;
		}
		if (tickOffset % 240 == 0) {
			var enemy = new Enemy();
			enemy.x = 2048;
			enemy.y = 2732 - enemy.height / 2;
			self.addChild(enemy);
			obstacles.push(enemy);
		}
		if (isGameOver) {
			LK.effects.flashScreen(0xff0000, 1000);
			LK.showGameOver();
		}
	});
	stage.on('down', function (obj) {
		hero.jump();
		var bullet = new HeroBullet();
		bullet.x = hero.x;
		bullet.y = hero.y;
		self.addChild(bullet);
		hero.bullets.push(bullet);
	});
});

===================================================================
--- original.js
+++ change.js
@@ -46,14 +46,19 @@
 var Enemy = Container.expand(function () {
 	var self = Container.call(this);
 	var enemyGraphics = self.createAsset('enemy', 'Enemy', .5, .5);
 	self.speedX = -0.6;
-	self.speedY = 0.6;
+	self.speedY = 0;
+	self.verticalMovementRange = 50;
+	self.verticalSpeed = self.verticalMovementRange / (5 * 60);
+	self.verticalDirection = 1;
+	self.initialY = 0;
 	self.move = function () {
 		self.x += self.speedX;
-		self.y += self.speedY;
-		if (self.y < 100 || self.y > 2632) {
-			self.speedY *= -1;
+		if (self.initialY === 0) self.initialY = self.y;
+		self.y += self.verticalSpeed * self.verticalDirection;
+		if (Math.abs(self.y - self.initialY) >= self.verticalMovementRange) {
+			self.verticalDirection *= -1;
 		}
 	};
 });
 var Game = Container.expand(function () {