"Well," said Frank slowly, "I suppose that the dirt brought down by the river and spread out over it makes it richer."
"Where does that dirt come from?"
"Way up the river."
"If I should call the bottom land a flood-plain," said Uncle Robert, "would you know why?"
"Oh, I know," said Donald. "Because the water covers it when there is a flood."
"Now what made that flood-plain?"
"Wasn't it always there?"
"No," said Uncle Robert. "The river made it."
"How could the river make the flood-plain?" asked Susie.
"Why, you told me a moment ago that the river brought down great quantities of dirt and left it all along the shores," said Uncle Robert.