Laozi once visited the ailing sage Shang Rong to seek his wisdom on philosophical matters.
Shang Rong advised him to dismount his chariot when passing his hometown. Laozi asked, "Does this signify that one should not forget one's roots?"
Shang Rong nodded and added that when passing an ancient, tall tree, one should walk quickly with small steps. Laozi responded, "Do you mean we should respect our elders?"
Shang Rong nodded again, then opened his mouth and asked, "Is my tongue still there?"
"Yes," Laozi replied.
"Are my teeth still there?" Shang Rong continued.
"No," Laozi answered.
"Do you understand the meaning?" asked Shang Rong.
Laozi said, "Are you showing me that the teeth are lost early because of their hardness, while the tongue endures because of its softness?"
"Exactly!" exclaimed Shang Rong. "You already comprehend all the principles of the world."