One leaf knows autumn, from "Huainanzi·Shuo Shan Xun": "Seeing a fallen leaf means you know the end of the year." Tang Geng's "Wen Lu" in the Song Dynasty quoted a Tang Dynasty poem: "The mountain monk can't count Jiazi, and one fallen leaf knows the world's autumn."
The same as "seeing the subtleties and knowing the work".
I don't know when I started to realize that I had a bad habit of "dog chasing mouse".