The great Doctor of the Church St. Augustine of Hippo spent over 30 years working on his treatise De Trinitate [about the Holy Trinity], endeavouring to conceive an intelligible explanation for the mystery of the Trinity.
Augustine was walking by the seashore one day contemplating and trying to understand the mystery of the Holy Trinity when he saw a small boy running back and forth from the water to a spot on the seashore. The boy was using a sea shell to carry the water from the ocean and place it into a small hole in the sand.
The Bishop of Hippo approached him and asked, “My boy, what are doing?”
“I am trying to bring all the sea into this hole,” the boy replied with a sweet smile.
“But that is impossible, my dear child, the hole cannot contain all that water” said Augustine.
The boy paused in his work, stood up, looked into the eyes of the Saint, and replied, “It is no more impossible than what you are trying to do – comprehend the immensity of the mystery of the Holy Trinity with your small intelligence.”
The Saint was absorbed by such a keen response from that child, and turned his eyes from him for a short while. When he glanced down to ask him something else, the boy had vanished.
Some say that it was an Angel sent by God to teach Augustine a lesson on pride in learning. Others affirm it was the Christ Child Himself who appeared to the Saint to remind him of the limits of human understanding before the great mysteries of our Faith.