This character's written form began with the drawing of a child's laughing mouth. Although the happy corners of the mouth have disappeared, 口 still retains the basic shape of the mouth in its current square form.
口 has many derived meanings such as "opening", "mouth of a river" or "door". It also forms the radical component of many Chinese characters such as 名 (name), 吃 (eat), and 号(number).
in Chinese culture, when we ask the question "How many people are in your family?", we say: "How many 'mouths' (口) are in your family?". And we reply with "We have x-numbers of 'mouths'". "Population" is translated to "Human Mouths" = "人 口". There are 1.3 billion people in China, or 1.3 billion mouths (口) to feed in China.