There is the famous story of St. Augustine walking along the beach contemplating the nature of the Trinity and watching a young man fill a bucket or a shell with water and dumping it in a hole he had dug. Over and over again he walked to the ocean a put little quantities of water in the hole.
"See that ocean?" he asked. "I'm going to put it in this hole I dug."
Amused, the saint replied, "You can't fit the ocean into that hole!"
To which the boy replied, "And neither can you understand in your mind the mystery of the Trinity."
With that the young man disappeared actually having been an angel sent to give the saint an insight into the depth of mystery.