按图索骥 — “To look for something according to a fixed pattern or existing clues”

Sun Yang, a native of the Qin State during the Spring and Autumn Period, was reputedly the most famous horse appraisal expert in ancient China. He could discern the quality of a horse at a glance. Because the legendary Bo Le was said to be the deity in charge of horses in heaven, people called Sun Yang "Bo Le."

It is said that Bo Le compiled his rich experience in identifying horses into a book called "The Classic of Horse Appraisal." In this book, he described the characteristics of various swift steeds and included numerous illustrations for people to use as a reference.

Bo Le had a son whose intelligence was quite poor. After reading his father's "Classic of Horse Appraisal," he also wanted to go out and find a swift steed. He saw that the book stated: "The main characteristics of a swift steed are a high forehead, large eyes, and hooves like stacked blocks of wine yeast." So, he took the book and went out, intending to test his own eyesight.

After walking a short distance, he saw a large toad. He quickly caught it and reported to his father, "I found a good horse! It's quite similar to what your 'Classic of Horse Appraisal' describes, except its hooves are not like stacked blocks of wine yeast!"

Bo Le looked at the large toad in his son's hand and couldn't help but feel both amused and exasperated. He humorously replied, "This 'horse' loves to jump; there's no way to ride it!"