This story tells that Wang Xianzhi, the son of Wang Xizhi, dyed three jars of water with ink in order to surpass his father in practicing calligraphy every day. In this way, the water quality in Wang Xianzhi is improving day by day. One day, he wrote a word "big" on the paper. Wang Xizhi saw that adding a dot on it became the word "Tai". Wang Xianzhi's parents read the words and said, "My son has been practicing for so long, and finally there is something (written by Wang Xizhi).
This is the origin of this sentence.