根据下面短文回答下列问题。 How men first learned to invent words is unknown; in other words, the beginning (起源) of language is not clear. All we really know is that men, unlike animals, somehow invented certain sounds to express ideas and feelings, actions and things, so that they could communicate (交流) with each other, and that later they agreed upon certain signs, called letters. These letters could be put together to stand for (代表) those sounds, and could be written down. These sounds, whether spoken or written m letters, we call words. They bring up ideas before our minds.