When I add a sprite to my game in the update function, the sprite is displayed, but the kill() method doesn't work. I suspect this is because the game property of the sprite is set to null. If I add the sprite in the create function instead, the game property is set correctly and kill() works just fine. Should I not be adding sprites in the update function? If I need to add all my sprites in create, what's the recommended way of keeping them hidden until I want them to be displayed?