One Father of the Church, St. Augustine, studied the mystery of the Holy Trinity and although he was at his wits’ end, he still could not comprehend it. One day he was walking on the sandy beach by the ocean. There churned in his mind the mystery of the Holy Trinity. He was talking to himself: “One God, but three Persons. Three Persons–not three Gods but one God. What does it mean? How can it be explained? How can my mind take it in?” And so he was torturing his mind and beating his brains out, when he saw a little boy on the beach. He approached him to see what he was doing.

The child had dug a small hole in the sand. With his little hands he was carrying water from the ocean and was dumping it in the little hole. St. Augustine asked, “What are you doing, my child?” The child replied, “I want to put all of the water of the ocean into this hole.” Once more St. Augustine asked, “But is it possible for all of the water of this great ocean to be contained in this little hole?” And the child asked him in return, “If the water of the ocean cannot be contained in this little hole, then how can the Infinite Trinitarian God be contained in your mind?”
Quoted from www.gocanada.org/Catechism/cathholy.htm