Thousands of years ago, the teapot had no mouth, so the water came out where it came in. After that, the mouth gradually formed, and this form of big in small out has been handed down to the present day. The main reason is that water is not easy to work together, and heat preservation. In fact, we still have the earliest usage and name of pot. "Warm pot", "chamber pot" are also where into which out, and called pot, is to retain the previous concept of pot.