根据下列材料回答下列各 题。

For some time past it has been widely accepted that babies and other creatures learn toDothings because certain acts lead to“rewards”；and there is no reason toDoubt that this is true．But it used also to be widely believed that effective rewards，at least in the early stages，had tobeDirectly related to such basic physiological“drives”as thirst or hunger．In other words，ababy would learn if he got food orDrink or some sort of physical comfort，not otherwise． 

It is now clear that this is not so．Babies will learn to behave in ways that produce results in the world with no reward except the successful outcome． 

Papousek began his studies by using milk in the normal way to“reward” the babies and soteach them to carty out some simple movements，such as turning the head to one side or theother．Then he noticed that a ba]by who had had enough to Drink would refuse the milk butwould still go on making the learned response with clear signs of pleasure．So he began to studythe children’s responses in situations where no milk Was provided．He quickly found that children as young as four months would learn to turn their heads to right or left if the movement “switched on”aDisplay of lights-and indeed that they were capable of learning quite complextul"11s to bring about this result．For instance，two left or two right，or even to make aus many asthree turns to one side． 

Papousek’s light Display Was placedDirectly in front of the babies and he made the inter-esting observation that sometimes they would not turn back to watch the lights closely althoughthev would“smile and bubble”when theDisplay came on．Papousek concluded that it was notprimarily the sight of the lights which pleased them，it Was the Success they were achieving insolving the problem，in mastering the skin，and that there exists a fundamental human urge tomake sense of the world and bring it under intentional control． 

（　）According to the author, babies learn toDo things which__________．