On the night before your physics exam, you hear a banging on your door, and in walks Kelly. She says, "There's a big problem here. According to Newtonian physics, only external forces can cause the center of mass of a system to accelerate. But a car accelerates because of its own engine, so obviously Newton was wrong." She crosses her arms in a way that suggests that she is not going anywhere until she gets a satisfactory explanation. How can you explain Kelly's error to her in order to rescue Newton and get back to your studying?