But here's another possibility: the hoary old fable of St. Augustine by the seashore. This baby has been making the rounds for centuries, and is well-enough-known to be the subject of several medieval paintings. You might find it useful
One caveat, however -- the story is plainly false. It comes from the Golden Legend, a collection of wildly imaginative, and generally spurious, vitae sanctorum. Please don't try to pass it off as a true story about one of the greatest theologians in history.
That said, here is William Caxton's translation:
... I will set herein one miracle, which I have seen painted on an altar of Saint Austin at the Black Fri[a]rs at Antwerp, howbeit I find it not in the legend, mine exemplar, neither in English, French, ne in Latin.
It was so that this glorious doctor made and compiled many volumes, as afore is said, among whom he 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 tbe sea-side in Africa, studying on theTrinity, 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 Augustin 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.To be honest, this story has a major flaw: it is very nearly the opposite of Augustine's "psychological" argument for the Trinity, which (so far as we grasp it, which isn't far) proposes that the human mind itself offers various operational "trinities", such as memory, understanding, and love, which serve as a model of the divine Trinity; and further that the human mind can begin to perceive the Trinity precisely because it retains the imago Dei. At least that's what we think he meant. (To see if we're right, check out De Trinitate. It may helpful to start with the synopsis in Book 15 and work backward.)