Where the shape comes from
A boundary closed on all four sides — city walls seen from above, or the line drawn round a piece of ground. It never stands alone but always encircles something else, and what it encircles is what the character is about: the population inside the borders in 国, the plants inside the fence in 园, the eddy turning on itself in 回, the prisoner in 囚. Written large it is this radical; written small it is the mouth 口.