Shi: In ancient times, there was a crossbar in front of the car for people to hold on to when the car bumped. In most cases, it was useless, but it would not work without it. It is a metaphor for not showing off one's appearance and showing one's sharpness. Be honest and do your duty. Su Shi's father must have hoped that Su Shi could be a down-to-earth and low-key person who was useful at critical times, instead of being an arrogant and high-profile snake oil. This is the best state of being an official.