By nature, mammals, including human beings, are playful creatures. Most of our childhood life is spent by playing games. Playing, in fact, is the most efficient form of active learning.
In essence, we live in the world, where we do not know what the actual reality is. In fact, we can only know the world, through our experiences and build these experiences to model our reflections of the world in our mind.
Our experiences, in fact, is our reality. Our experiences have become parts of our memories and which will be later used to make decisions. Therefore, we live in 2 parallel worlds, one of which is our mental projection of the reality and the other is the actual reality, which we do not know. According to our experiences everyday, we are constantly changing our mental model to understand the real world. Whenever there is a difference between our mental model and the real world or whenever there is something new which doesn't exist in our mental model, we adjust our mental model to fit for the reality; this is what we call the 'learning'.
Learning is not easy effort. It involves identifications, classifications, connections, associations and recognitions. Whenever we come across new things, we need to identify them by using our existing mental model. To put it simply, we need definitions for our new things. Then, we need to categorize them, and put them under some categories. Then, we need to connect them to other similar or different things. Then, we associate them with something vivid. (For example, when we hear a word 'Apple', it is interesting to see what has appeared in our mind: the mental image in the shape of apple or the word itself. And if we are bilingual, what we first describe the apple, the mother language or the second language.) And finally, we will be able to recognize them and they have become part of our mental model. At this point, the learning is complete.
I think what makes human beings different from other mammals or humanoids, such as Chimpanzee, is not our intelligence; it is our ability to learn. As far as I can tell, the ability of human to learn is boundless. We can learn languages, mathematics, arts, music, cooking, dancing, fishing, swimming, flying, martial arts, sports, and well, what not. It is not because we, human beings, are more intelligent than other mammals, it is more likely because we can more quickly change our mental model to adapt the new situation. As far as I can feel, it is the greatest gift given by the evolution and natural selection.
Playing is, in fact, a part of process of building mental model by simulating the reality. Playing, however, has some limits; it is the simplified version of reality by predefined rules and goals. It is in fact the learning tool which allows us to learn more actively.