四维空间

• 囗(frame; surround)RADICAL
• 儿(person; foot)ASSOCIATIVE

Four sides of a square frame surround two legs — the ancient form of the number four.

• 穴(row; gap)RADICAL
• 工(craftsman; work)PHONETIC

An empty cave with nothing inside is just hollow space.

• 門(door; leaf)RADICAL
• 日(sun; light)ASSOCIATIVE

A beam of sunlight shining through a crack in the door creates the empty space in between.