"Business is the main thing here, anyhow," I'd answer.
"And what's 'business'?"
"Well, it's—er—it's like this: Suppose you, for instance, were to go and catch a great many flies—"
He smiled dreamily
The fish would look pleased and smile dreamily.
"But then not eat them, mind you."
"Not eat them?"
"No, but put them all out on a bit of flat rock, for a counter, and 'sell' them to other fish: exchange them, I mean—for shells, let us say, if you used shells as money."
The fish would look puzzled. "But what for, my dear sir?" he'd inquire. "What would I do with shells?"