Even a child in kindergarten can be the smartest person in this world by doing this action. What is it?

Answer:Remembrance of death and preparing for it. The Holy Prophet of Allah sallallahu alaihe wasallam has said, "The most intelligent person is the one who often remembers death. Constant remembrance of it prevents one from evil and sin and motivates him towards good morals. The Holy Prophet sallallahu alaihe wasallam said, "The remembrance of death defeats joy." (Ibn Majah)