Quarks in hadrons, electrons in metals, neurons in brains, cars on streets, dealers in stock markets, … whatever the fundamental elements of the world may be, most questions we raise in our lives are many-body problems where many elements interact with each other. The many-body problems have not been solved and will not be solved in general. The world is so full of many-body problems that we cannot even pretend these problems are unimportant details of nature. Modern computers expand, by a great deal, the size of tractable many body problems. By this, we can further approach the true nature of the world. However, we will never get an ultimate understanding unless we cast the computer output into simple sentences in our own human language. In other words, the last step is ultimately up to us.