The light in the sky and the shadow of clouds in the water.
The source of living water originally meant that the more you read, the clearer the truth. Now also refers to the motive force and source of things.
Idiom allusion: When a square pond is opened for half an acre, the sky clouds drift. Ask the canal to be so clear, because there is running water at the source. Song Zhuxi's poem "Random Thoughts on Reading"