In A Brief History of Time (Bantam Books, 1988), Stephen Hawking tells 
the story of an elderly woman who confronted Bertrand Russell at the end 
of a lecture on orbital mechanics, claiming she had a theory superior to 
his. "We don't live on a ball revolving around the Sun," she said, "we 
live on a crust of earth on the back of a giant turtle." Wishing to 
humor the woman Russell asked, "And what does this turtle stand on?" "On 
the back of a second, still larger turtle," was her confident answer. 
"But what holds up the second turtle?" he persisted, now in a slightly 
exasperated tone. "It's no use, young man," the old woman replied, "it's 
turtles all the way down."


Stephen Hawking in BriefHistoryOfTime starts with the same anecdote.
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."