Teachable Moment: What do we really know and how solid is the foundation of that knowledge? That's the fundamental question of all science and, as Hawking so brilliantly realized, the turtle story makes this question tangible and fun to think about.
In Stephen Hawking's 1988 book A Brief History of Time, he starts with the following story:
A well-known scientist (some say it was Bertrand Russell) once gave a public lecture on astronomy. He described how the earth orbits around the sun and how the sun, in turn, orbits around the center of a vast collection of stars called our galaxy. At the end of the lecture, a little old lady at the back of the room got up and said: "What you have told us is rubbish. The world is really a flat plate supported on the back of a giant tortoise." The scientist gave a superior smile before replying, "What is the tortoise standing on?" "You're very clever, young man, very clever", said the old lady. "But it's turtles all the way down!"
References: Hawking, Stephen (1988). A Brief History of Time. Bantam Books. ISBN 978-0553053401.