play() {
    var entities;
    var i;
    var key;

    // Already playing.
    if (this.isPlaying || !this.hasLoaded && !this.isLoading) {
      return;
    }
    this.isPlaying = true;

    // Wake up all components.
    for (key in this.components) {
      this.components[key].play();
    }

    // Tell all child entities to play.
    entities = this.getChildEntities();
    for (i = 0; i < entities.length; i++) {
      entities[i].play();
    }
    this.emit('play');
  }
Start dynamic behavior associated with entity such as dynamic components and animations.
Tell all children entities to also play.