For example, imagine that your little sister wants to move her bed from one corner of the room to the other. She gives the bed a light push, but the bed does not move. She gives it a stronger push, but it still does not move. She gives it her strongest push, and still nothing happens. She calls you for help. Only when both of you push together as hard as you can does the bed finally start to move. Why? Why did the bed not move when your sister pushed it by herself? What kept it from moving?