"Continents," said Frank.
"Yes," said Uncle Robert. "We live on one of them."
"The continent of North America," said Donald.
"Our island rises right out of the river," said Susie.
"Rock and water make only a part of our world. We live on the firm earth. But we live in something. Indeed, we live at the bottom of a great, deep ocean, deeper than the water ocean, and broader than the rock and water surface taken all together."
"We live at the bottom of an ocean!" said Donald in surprise.
"Now you are joking, Uncle Robert," said Susie. "If we lived on the bottom of an ocean we should all drown."
"Fish live in the ocean, and we live in an ocean, too—a very deep one, how deep no one really knows. It may be a hundred, or hundreds of miles deep. We see a part of the surface of the earth and of the water, but no one has ever seen the surface of the mighty ocean in which we live."
Susie and Donald were puzzled. Frank's face lighted up as he said:
"I think you mean the air, Uncle Robert."