Using Driven Keys
It would be a pain to animate EVERY ITEM EVERY TIME it moves. Animating keyframes is great, but often things are reacting to other things. When that is the case and especially if it's reoccurring, it's better practice to consider using driven keys. Driven keys are animation's If/Thens. In the case of this tutorial, if the ball approaches the door, then the door opens. If the car approaches the garage door, then the garage door opens.
The concept and impact of driven keys is really significant and powerful.