The Buddha was occasionaly challenged by the sophists of his time to prove the existence of the other shore, as he used to refer to the unconditioned reality of the Buddhas.
He used the story of the turtle and the fish living in the same pond.
One day, the turtle decided to go for a walk on the dry land. The fish were wondering whatever had happened to him. As far as they were concerned, he just vanished from reality.
When the turtle was done walking, he returned home, that is to the pond.
The fish were delighted to see ole' Mr. Turtle re-appear.
"Mr. Turtle," said they, "what on pond has happened to you? You just vanished!"
"Oh, I just took a walk on the dry land," quoth the turtle.
"What, pray tell, is a 'dry land'?" inquiring minds wanted to know.
"Oh, that's the world outside the pond."
"World outside the pond? Wow, tell us more!"
"Well, lessee," said the turtle, "it is covered with green grass..."
"Green grass? Get out! How can a world be covered with green grass? Water would destroy it!"
"But the dry land is not filled with water," the turtle tried to explain. "It has air, and sky above, and beautiful clouds, and..."
"Not filled with water? Who do you take us for? You think we're stupid, don't you, you Old Foolish Turtle."
"No, really, the world is..."
"Oh, shut up, Turtle! The world you are describing is impossible! How could you move in it if it weren't filled with water. It does not exist!"
"Whatever," said Mr. Turtle, and off he went for another walk.