I used an image in my entry about the Post Communion prayer for Mass of the Most Holy Trinity which drew comments in email.
The image, a painting by Botticelli, depicts the legendary meeting of St. Augustine with a boy along the seashore.
The story goes like this, according to this version from the Golden Legend by Jacobus de Voragine (+1298) as translated by William Caxton (+1492).
Saint Augustine made a book of the Trinity, in which he studied and mused sore in his mind, so far forth that on a time as he went by the sea-side in Africa, studying on the Trinity, he found by the sea-side a little child which had made a little pit in the sand, and in his hand a little spoon. And with the spoon he took out water of the large sea and poured it into the pit.
And when Saint Augustine beheld him he marvelled, and demanded him what he did. And he answered and said: I will lade out and bring all this water of the sea into this pit. What? said he, it is impossible, how may it be done, sith the sea is so great and large, and thy pit and spoon so little? Yes, forsooth, said he, I shall lightlier and sooner draw all the water of the sea and bring it into this pit than thou shalt bring the mystery of the Trinity and his divinity into thy little understanding as to the regard thereof; for the mystery of the Trinity is greater and larger to the comparison of thy wit and brain than is this great sea unto this little pit. And therewith the child vanished away. Then here may every man take ensample that no man, and especially simple lettered men, ne unlearned, presume to intermit ne to muse on high things of the godhead, farther than we be informed by our faith, for our only faith shall suffice us.
This story has garnered many depictions in art. These are just a few.