口
kuchi
One mouth becomes the entrance and the service window
The picture A small opening: when it opens, inside and outside connect, and things go in and come out through it. 口 is an opening things pass through; 目 is the eye that only looks, with nothing passing.
Where the picture shows up in 口
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the opening in a face
口を大きく開けてください。
kuchi o ōkiku akete kudasai
Please open your mouth wide.
Someone lies back in a chair with their mouth open as a small mirror goes in
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the opening you go in by
入口はあちらです。
iriguchi wa achira desu
The entrance is over there.
People are drawn one after another through a door in the side of a building
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the opening you come out of
出口で待っています。
deguchi de matte imasu
I'll wait at the exit.
A stream of people pours out of one door in the station wall
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a small gap words cross
窓口で聞いてください。
madoguchi de kiite kudasai
Please ask at the counter.
Papers and words pass back and forth through a slot under a glass partition
窓口 is 'the hole in the window': a service desk drawn as an opening.
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the number of mouths coming and going
この町は人口が増えています。
kono machi wa jinkō ga fuete imasu
The population of this town is growing.
On the town map, one more eating mouth is stamped in, then another, and the count rises
人口 is 'people's mouths': heads counted as mouths to feed.
All of it in one picture
口 is one opening things pass through. Put it in a face and it is a mouth; put it in a building where people go in and you get 入口, the entrance; where they come out and you get 出口, the exit. Cut a small gap in a window so words and papers can cross and you get 窓口, the service counter; count the mouths that come and go in a town and you get 人口, its population. Face or building, the same hole is there in the picture.