The wheel instances are placed on the Timeline of the car parent movie clip.
To make both wheel instances spin, you set up a motion tween that rotates the wheel symbol. Even after you change frontWheel and backWheel, they continue to be affected by the tween on their parent movie clip, car; the wheels spin, but they also move with the parent movie clip car across the Stage. To add wheels that rotate, you create a movie clip for a car wheel, and create two instances of this movie clip, named frontWheel and backWheel. Then you place the wheels on the car movie clip’s Timeline—not on the main Timeline. As children of car, frontWheel and backWheel are affected by any changes made to car; they will move with the car as it tweens across the Stage.
You can use actions to send messages from one Timeline to another. The Timeline that contains the action is called the controlling Timeline, and the Timeline that receives the action is called the target Timeline. For example, there could be an action on the last frame of one Timeline that tells another Timeline to play. To refer to a target Timeline, you must use a target path, which indicates the location of a movie clip in the display list.