This is a joke, apparently a true story, about Wang Xizhi 王羲之, the Sage of Calligraphy 书圣.
The story goes that when Wang’s son, Wang Xianzhi 王献之, who later went on to be a famous calligrapher in his own right, was young, his father would have him practice for hours on end.
One day, Wang Xianzhi wrote long enough to use up three large jars full of water, and then couldn’t practice any more. He brought the results to his father, hoping for praise and encouragement. Wang Xizhi wasn’t impressed, however, and looking at one character in particular, 大, said it was unbalanced and too loose on the bottom, took a brush, and added a small dot, turning it into the character 太.
Later that evening, Wang Xianzhi showed his work to his mother, and asked her if he was as good as his father yet. She went through the characters one by one, and being familiar with her husband’s calligraphy, was able to tell that her son still had a long way to go until he matched his father in skill.
But when she got to the character 太, she said, “This one is okay, it looks like a little like your father’s.”
It doesn’t translate so well into English, but in Chinese it’s funny because she says 唯有一点像王羲之. 有一点 means a little, and the small dot stroke that Wang Xizhi added is also called a 点. The sentence, therefore, can mean either, “It only looks a little like your father’s”, or, “Only the dot looks like your father’s”.