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!"
-- Stephen Hawking's 1988 book A Brief History of Time